I don't have any experience with 5x druid teams, but I have thought a bit about making one.

My guess is that you will find it easiest to level them all balance until 60. Pre-outlands instances frequently do not require a true tank, and when questing having 1 feral/resto will actually be a disadvantage since you'll hardly take damage or need to tank anyway. I can't be sure, but I think a 5x starfire+5xmoonfire opening burst should down most any elite.