It's going to sound like a copout answer, but you should play whatever mixture makes you happy The time to level from 0-70 really isn't that long if you utilize RAF to 60.

I started with RAF because I had a horde moonkin and wanted an alliance one. Once I realized that I actually could run two clients on my laptop I said wth, I'll try three. I have been very satisfied with my three moonkins and they're a ton of fun. However, as is the case with most who get into multiboxing at a significant level, I soon felt the itch to go to 5.

If I was you, then I would work on your druid and warrior and see if you like the mixture of tanking/healing. If you can handle that part, and enjoy it, then I think adding 3 DPS will be extremely easy.

I'm a bit concerend that you have a 70 and a 71 immediately available. I don't know if they're on one account right now and you plan on transfering them to the new one, but many new multiboxers try to use their friends account. Even if that friend gave you the account, unless your name is on the account, then you're in danger of getting both banned. If that's the case, then I would definitely only play the account with your name on it... which would allow you to start two new toons of whatever class you choose!