You can always change classes so I am not sure I am seeing what you are really asking here. For RAF'ing you need to have -very- similar levels in order to get the benefits, but what class they are does not matter. When all said and done however, you can make a team of three from one character from each account. If you are willing to pay a bit of cash and have room on your accounts I believe Blizzard will even let you move characters between accounts you own (Been a while since I looked at that so I might be wrong).

If one of these doesn't answer your question, feel free to ask again and Ill try again