Best advice I can give on this is to "trust your ears".
Mix them, then listen..

To be honest, final levels fall on the mastering engineer.
There are easy ways to help balance the mix, such as using PGPeakLimit in the final output and visually balance things.
Also the PGRTA helps get a look at the sonics.

http://musicmastering.org
Check the premastering page.


I do not work here, but the benefits are still awesome
Make your sound your own!