pva bags dont need specific baits in, you can use what ever your using for the type of fishing your doing. e.g bread for roach, pellets and boilies for carp!
theres no set ideas or rules use what ever bait you want, you just need to remember that some bags have wholes in, so small pellets, crumb, or small particles could come out. but you can get all different sized and shaped bags and some wont have holes in, buy the ones which will suit your needs! just make sure the baits you put in arnt moist as the bag will start to dissolve. this is why they arnt always pratical if its raining! i dont know much about this maybe one of the carpers may no about it, but i believe you can actually get special liquids which will not dissolve the bag?
i could be very wrong but if your using bread in a pva bag on a stillwater with bread in it, the pva might dissolve on to the bread leaving a sludge of pva ontop of it??
so like i said fish them to suit your needs...