i'd agree with most here.

as you increase the number of different classes, you increase the complexity

if you throw melee into that.. it gets even worse

i'd recommend gettign 3 of the same ranged /caster types.. and just get them to 20. this will get your brain use to the mindset of multiboxing without worrying about the problem of multiple keyboard types. then you'll start getting crazy with your macros... and by the time you are done, you would be better able to handle multiple classes

here's a group composition thought:
tank/heal/dps ... warrior,priest,3xmage

or you could try warlocks... where the voids can double for tanks
4xlocsk,1xpriest

loads of combinations... have fun and don't be afraid to reroll