BTW, the pic above is the full 6th edition rulebook cover front to back.
Allies are taken as a seperate detachment from your regular force, meaning that you must take 1 HQ and 2 Troop from your regular chosen codex before you take a detachment of anything. Once that is done, you can take 1 HQ and 1 Troop choice of an allied detachment, and once complete it opens up an additional 1 Elite, 1 troop, 1 Fast Attack, and 1 Heavy from the allied army codex. Note that as far as we know you cannot take special characters with an allied detachment.
There are some obvious restrictions.... These come in the form of three levels of allies. Lets look at it
via the Video release from GW... Glimpses of the Rulebook Allies Page
Units in your army treat allies of Convenience as enemy units that cannot be charged, shot, targetted with psychic powers or have templates or blast markers placed on them. However, if a psychic power, scatering blast weapon or ability that effects an area hits soem of these allies of convience they will be affected along with any friendly or enemy units.
Can't benefit from the Warlord Trait of an allied character
Cannot be joined by an allied Independent character
Now that is directly from the video were we get to see the page of the rulebook. I am sure there are more rules we cannot see there.
This is what the rumors say
Battle Brothers: Good can join each others units with IC's and cast psychic powers as if they were friendly.
Allies of convienance: Cant join units or cast friendly powers
Desperate Allies: Have to roll if within 6" of an allied unit. if they roll a 1, they cannot do anything.
While the selection of allies appears to be huge at first glance, the chart I will be bringing up will shot it to be only viable with certain armies. Lets look at who is probably getting what..This chart is out of the rulebook.
Lets get into the positive, since I have been list making now for a bit.... Double FOC and Allies are in the same boat as they allow a greater flexibility in army types we will see on the tabletop. Its not all about the cinematic, although allies makes more sense this way, than it does limiting everyeone to one codex. The armies we will start seeing will contain an amazing diversity with these new rules. Already I have seen what my guard lists will like, and those lists are much different than they were only a few weeks ago. The amount of flexibility is amazing and fun and breathe some new life into the game.
Balance. I have said it before, that I truly beleive that allies and double FOC are the way to go. You did not see it much in 5th edition becuase the point level was above 2500pts before you could double your FOC. Now with it lowered into playable games, you will get so much more life out of your armies, tactical holes in it will be able to be possibly filled. There will always be some weakness to your list, after all you only have as many points as you did before.