dont forget if you roll 5 dks you can pretty much get them to lvl 61/62 ish right thru the starter zone for them, plus they're a lot of fun!

i recommend that you take at LEAST 1 pally up to lvl 60... so you can run 4 dks 1 pally, which seems to be the latest greatest setup

if you intend to do rogues i'd run one druid and 4 rogues, but you've already got a druid, so perhaps another healer for that group, priest comes to mind... people like to come after priests out in the open, so he'd be the perfect bait to pcik off the poor sucker who tries to ggank you lol.

mages are good to have too, lots of fun doing things like pvp with them. if you go with warlocks, they can be a lot of fun too, they look hella cool in demon form * 4 or 5 lol

ima agree with kasurahama on this one last point, i would level 5 of everything except rogues.

5 pallys pre cata was a ton of fun too, and they're fun to level sooo they might be a good idea as well !