It can be done but you need to do it because you want the challenge and you want to have the maximum effective PvE group.

I run with 4 different classes and it's not that hard to treat a destruct warlock as a third mage.

There are definitely times I -wish- my second mage was a shaman.

If your goal was to get 9 diff classes to level 70 in the least amount of time possible, this is probably a bad way to do it. A better way would be take your level 70 and powerlevel 4, and repeat as necessary.