Here is some more ally info
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.
Necrons Allies:no battle brothers. allies of conveniance with chaos marines, gery knights (!!), tau and orcs. cant ally with deamons, any eldar or nids. all the rest are desperate allies. Desperate allies with Templars and Blood Angels