What's the size of the venue?

When faced with a lack of gear, I go in with the mindset of bringing everything down, as opposed to bringing everything up. But since the choir is not singing with the band, I would suggest maybe 3 mics are more than enough (especially for recording). The only thing as far as height goes, would be to make sure the choir is singing INTO them. Otherwise you can lose clarity and definition and it will sound as if each person had gum in their mouth while singing. :D However not all choirmasters are happy with the added "decorations" you just placed in front of their choir.

My experience also tells me that in the mix, you'll always end up falling back on the main stereo pair as it will have a nice, rich, full sound. Maybe just one of the spot mics to bring up the basses and tenors a bit...

I dunno man, I hope someone will be able to answer a bit better. I have more experience singing in a choir, than recording it. :)

A couple of questions back at you, mainly so that we don't have too many assumptions here:
You want to record this 15-person choir while they are monitoring this MIDI tone generator through some stage wedges, correct?
Simultaneously, the choir (as well as the MIDI box) will need to be fed through the church's sound system to the congregation, correct?
What mics were you planning on using in what positions?
Is there going to be a soloist stepping out from the choir, or is it all ensemble arrangements?
How are the vocalists arranged, physically speaking?
Let us know. My first reaction would be to say that the fewer the mics involved, the better. But inquiring minds want to know...

Mark van J: Venue is about 60" long, stone walls, pews full of old women. From front speakers to the back row is about 40" though.
The system is installed in the Church, we use it on Sunday mornings and for occasional concerts. This gig is just a little bigger than usual.

I know what you mean about the "decorations". Thankfully this choir uses mics at their church when they sing. They use 3, in a straight line, though I only got a glance of that when we did a concert there before the mics were being moved.
Although, stereo pair? Which mics are those, or is it the L-R out on the box?

Lack of equipment is too ambiguous, now that I think about it it's more a lack of buses/aux sends/recording equipment, rather than being short of mix channels/amps.

Moonbaby: Yeah, should've mentioned that. Midi to the wedge. Thank you very much for mentioning that however, because I just realised my intended setup doesn't allow me to do that.
I know nothing about solos happening, I will likely find out during the gig. This is the norm. The lineup? I know nothing either. I'd imagine it's a 2 or 3 row box though I expect to be rearranging on the night. I wouldn't mind so much if I had time, but I'm going to have about an hour (if that) to soundcheck a band and a choir on 2 different setups - and depending on when the choir arrive...
This kind of thing happens a lot, because in Churches, we believe in Jesus - not rehearsals and sound checks.

Was going to use Shure PG58s as the front 3 (or maybe 2) which would do the bulk of the FoH job. The intended side mics are unlabelled. I think they're AKG. These were going to be say, 4" from the choir's 'edges' at about 60 degrees to them, more for acoustics in the recording rather than FoH output cause the speakers aren't that great. Should they be closer, nearer the front?

I take it that since the church is a traditional Anglican type affair, that there are no "stepped" choir boxes. In other words, each row is directly behind the next. OK, if you have 15 people in, say, 3 rows, that's 5 people across each row. That would make each row about 18 feet wide. Is that close? I would say that the Shures, even though they're the PG series, would be a better bet because they are designed for the voice.
WE have no real info on the other mics in the rig.
Then I would use 2 of the Shures, one on each side, out in front of the choir about 3 feet. Try to use mic booms so that you can get them up in the air.
Start you positioning with the mics up at least 6 feet, preferrably 7. Point them inward so that each one is aimed at the center of the rear row. You will have to play with this a bit, and you may find that cocking them downward a bit is helpful. This is how I've used SM58s in the past, and it's worked fairly well. You can TRY the 3rd mic in the middle, but it may cause too many phase cancellation issues. If you don't have the time to experiment, I wouldn't chance it. Besides, you still don't know whether you'll need a soloist mic...
Now to place the wedges. Some say that you'd aim these at the rear of the 58's, where the mics are least sensitive. BUT the mics are going to pick up the wedges no matter where you place them. And the null side of the mics is usually the worst sounding (off-axis). I would "single-point-cluster" the wedges in the front-and-center of the choir. You have to keep their level as low as possible to minimize leakage into the mics. But what WILL be picked up won't be as muddy in the mix.
As for the other mics as "ambience", how many tracks are you able to simultaneously record? If only 2, I hesitate to suggest anything. They may make things worse. I use a couple of tracks on an HD24xr, but I usually have those mics out in the congregation. If the loudspeaker system is as bad as you say it is, maybe you should skip that altogether.

I can record a max of 6 separate tracks although the crosstalk and noise level is far from great (varies from -70 to -50dB), and I'd prefer to use 5. The onboard has 2 separate inputs (each is stereo...the mixer outs are mono...) which gives me 4 + a stereo line or mono mic (which is louder) on the spare soundcard.
The loudspeakers aren't awful, they're about 15" wide and sound OK, until the volume builds up.

The choir will be flat, yes. There's roughly 20' available but thinking back to the gig we did at their church, their "stage" is about 15' at most. Maybe it's all small old women...

The wedges aren't wedges as such. Laid flat the speaker makes an angle of about 15 with the ground. I could prop them up on hymnbooks.
The "single point cluster" isn't a great option. There's a lovely flight of stairs in front of the choir. The choir could go back a bit (up to 10') but they can't come forward onto the stairs on account of the front pew being 6' away, so there's no space for wedges/mics. Stairs are about 10' wide.
[Edit: cleared this paragraph up]

So I use 2 PG58s where the dark blue "jelly beans" are, about 6 feet up?
Or the dark greys, higher up and pointing at the centre back row?
Or should I do dark blue + grey, or grey + blue, or all 3 blues?
With the dark blues, I'll base them on the stage, and swing them across so the mics are nearer centre.

Wedges could be placed at the black rectangles, or one at the light grey rectangle. If it seeps, it's no major worry. If and when they want clean, proper recordings, someone else can do them.
These recordings are mainly a courtesy to the choir, an experiment with the band and something which could potentially lead to doing real recordings.

Actually, what I was describing that I do with the 58s is to place them more where you have the "gray mics", maybe pulled a bit more inside.
BUT as long as you have your "blue mics" at least 9-10 feet apart (with a distance of 3 feet from the choir) you should be fine.
Something else to consider...
Using the "pencil mics" you said that you thought were AKGs in an ORTF stereo configuration in the center. Google this:
Innocent Ear....this is a UK-based recording service specializing in classical-style productions. Scroll down the left-hand listings to ORTF, check it out. This is a valuable resource for anyone interested in what you're trying to do, even if you don't use it for this gig.
Good luck!

Hmm...I'll do that then.
2 58s at 10' apart, 3' in front of the choir. I'll stick the wedge dead centre on the stairs.

Thanks for your help. Saved the recordings from a painful, phased-out death.

Well this has now been and gone and the recordings turned out OK, save for a few things.

Wedge ended up almost touching the front pew, so the singers could hear it more evenly. Something I never thought of.

Five minutes into the (20 minute long) soundcheck the power failed. No problems with anything, just that the socket which our extension is plugged into happens to hang off the wall. Naturally, it cut once and required some "persuasion". Also cut again today before the service. Grr...
Lesson: Always check the connections are in right.

Halfway through the gig I noticed the reason our drummer was complaining about the monitor being quiet...sometime in between the rehearsal and gig, the Aux 2 feed had come loose. My forcing of it back in only screwed up an already badly done song and caused a few onstage pops, so no worries (the speaker still works).
Lesson: Always check the connections are in right.

The recordings turned out 90% fine, haven't sent the choir their CD yet though. And the drummer was wanting me to email him all the recordings from it. Like I would email anyone 100MB of mp3s.

Funny aside: my friend works for Glasgow City Council. For some reason they have a PDF circulating which is roughly 2.6 GB in size. 9000 odd pages which took 2 days to print. And one guy tried to email it to himself as a backup.