Peach seems to work great on everything I've tried it with, especially pork shoulder, and I actually would have it green rather than so dry that the bark is starting to come off. With greener wood you won't need to use as much and the smoke will last longer. For pork butt or brisket, I often use a few peach chunks with one or two chunks of hickory/oak, the amount and size depending on the age/dryness of wood. I often will use three chunks down in the coals and three on top.
Regarding what's "seasoned", I read just the other day where Chris Lilly (of Memphis in May and Big Bob Gibson's BBQ fame) considers anything THREE months or older as being "seasoned". Lots of folks would consider six months necessary to be seasoned, but regardless, my experience is that seasoned wood is only REQUIRED for wood burning smokers. Now I'm not mimimizing the mistake of oversmoking. However, that can be done in any cooker, no matter whether using seasoned wood or not.