Well, to start, I'd do your (a5) priest, (a4) pally and the 3 mages up to the 30's, then you can switch the mages out for shamans if you wanted, keeping the pally and maybe priest.

next, (a5) warrior (a4,3,2) Locks and (a1) priest. With the voidwalkers, you don't have to worry about aoe tanking, they'll help a lot. Take them through WC or SFK and the priest will still be able to get a little xp. But the 20's toons will catch up to the 30 priest sooner or later