With regards to tank, get a paladin, much easier to box than other tanks and just as effective.

I have to agree with the above posters with regards to mix groups. they do require a bit more work than 4 shammie 1 prot pali but i found thats mainly time spent researching each classes dps and figuring out how to macro it.

And finaly on what classes to take if you decide to make a mixed Id really suggest dropping the mage, as they are so mana hungry even if they out dps the rest of your team(my mage never does) you lose the benefit from it by having to drink all the time. I have a mixed group using pretty much what you suggest and Im in the process of lvling a hunter to replace the mage, unfortunatly. and definatly keep a boomkin in! way too much utility to lose, there great off healers, off tanks, the only other class you could duel spec tank if needed in your group, they can battle rez and no end of other things that make then perfect for a mixed group. plus my booms DPs is aove my mages 99% of the time