The current rumors on how it is going to work is....
First you choose your 1 HQ+ 2 Troop
then if you are going to have an ally, you pick 1 HQ+1 Troop from the ally. Then you can take 1 Elite, 1 FA, and/or 1 HS. There is a rumor going around that the ally has to be less than 25% of your point total. It appears that you will not be able to attach an IC from one army to another or use a transport from another army (no Terminators in BWs or Space Marine Chaplains attached with Snikrot).
If this is true, I think IG is going to be the premier ally. Get a cheep HQ, Infantry Squad, then a Squadron of Armour.