Your first question is too subjective. The best answer is, put them where you like. Every nuance of you mix, such as panning, should be expresive of composition goal.

-5db is good. between -5bd and -3db, and keep your bit rate up. 24bit or 32bit. if you are recording in only 16bit you might want to consider closer to 0.

Drum Overheads :
Try to place them hard left and right , and then step by step try to get them simmetricaly closer to the center ,then look for that sweet spot your ears likes ...

Bass drum :
Start with placing it in the center to share the bass load on both channels

Guitars :
No rules at all dude

Bass-Guitar:
Center

Lead vocals :
Center

Backing Vocals :
Depends on how many backing vox you have there , try to imagine the whole backing vox singers on a stage or even draw a picture of it , with the lead vocalist in the center and create this image by taking some vox to the left and some to the right or center

I would go hard L & R with the drum overheads. Bass guitar & bass drum center. 2 guitars L and R (either full or partially, depending an what sounds better). Lead vocal center. B/U vocal, either center, or I might consider using a digital delay set at something like 10 ms and pan the original full L and the delay full R.

Experiment. Try using the delay on one of the guitars also to split L & R , and hten put the other one center.

"And how is it about mixing, does the ready mix have to go to 0db???
Or should i go to about -5db, and get the rest of loudness trough mastering??? "

Usually you get the mix up to 0 dB. You should get it sounding as close to the final master as you can. However, it doesn't make a big difference in this digital age - in mastering you have plenty of flexibility.