I am editing a documentary which will be finished to DVD.

My audio is OK from origin, being around -12db.

I am not sure how to finish my audio for DVD. There are interviews and bands playing. The interviews have the audio at a constant level, but the bands have a lot of peak variations.

I am not sure at how much dbs my final audio should be. I am also not sure if I should apply a compressor or a limiter in Soundtrack Pro so that all my audio is around the same level.

I think I should leave my audio levels at around -5 db, and limit it at -1db. But I really dont know if this is OK or not.

I appreciate suggestions.

It's entirely up to you, and it's also a pretty complex topic.
Just FYI, most professional DVDs peak at around -6 dB.
Jay Rose has a good book out on the whole post-production aspect of audio that might be a good reference for you.