Just a little unclear as to exactly what you are hooking up to what particular device. You can for the purposes of practice, as opposed to mixing, hook the interfaces line level output to your PA system going into a stereo line level input. This will not damage anything. If you want a more specific routing you need to ID your equipment. If you are askingif you can hook the line level outputs of your interface to a guitar amp the answer is that is not a good idea at all. A guitar pickup is high impedance but extremely low output as it is unamplified whereas the output of your interface has already passed through some type of amplification so the resultant signal is too "hot" for the preamp stage of most guitar amps and yes can/will likely cause damage.

Yes the reason I ask, is because we do not have a pa system, as our singer has bailed on us, And we are left with just four,,,, two guitars, one bass one drummer.

Again we want to record so that I can practice to guitar tracks instead of just trying to think I am playing what I need to play.

The equipment specifically is a tascam us1641.

It has 8 mic in 2 guitar in, 4 line level in. two line level out, headphones out.

We are specifically trying to record while playing, and listening. Yes we originally had the outputs of the tascam plugged into the amp ins.

like I said we ran from guitars to effects boards (not sure of make) to guitar in of tascam, out to amp ins one is a peavey bass amp, one is a hartke half stack.

Ok there are of course many different ways to record. Typically guitars are recorded by micing the amp the guitarist is using, this is because the amp has tone controls, offers different qualities of amplification, for example distortion or "crunch" and the speakers size and the amount it is driven all contribute to the sound a guitarist prefers. Another thing is that many pedal effects sound like crap when not processed through a guitar amp. So your plugging the pedal boards direct in is certainly one way to go but rather unusual unless you are recording drum tracks only and then the guitar tracks are considered "scratch" tracks (meaning they are disposable) and again typically if you were going to do that the individual players would be tracking through headphones, when there are more than one or two a headphone amp is usually employed.
I assume what you are trying to do is record practices, in which case have everyone play through amps and use one, two or three room mics to record. Back in the day that is how recordings were done. You will need to move the mics around to determine where you get the best mix of sound but this works pretty well in a decent enviroment. Another method would be to see if the amps have direct out or effect loop outputs if they do then you can run the guitar/effect into the amp and bring a 1/4" cable from the output on the amp to a line level input on the Tascam. Your other choice is to continue doing what you are doing because you don't have any money until you ruin the amps and have to come up with money to buy new ones. In any scenario it will always lead back to the fact you need to have the right equipment and having the right equipment will save you money in the long run. Hope that helps.