There were many rumours about how the force organisation charts would be modified. What is really interesting is the possibilities it opens up.
As with 5th edition rules, you're allowed so many Command, Troop, Heavy, Fast, Elite options for a codex. But then ... you are allowed to take a few options from another codex. Interesting huh?
Yes, you're allowed to take an allied army - a command, some troops (and other) options.
But you can't choose just any two options. There is a chart which allows you to look up two codexes. Some codexes work better together than others. Tyranids for instance don't work with anyone. Tau on the other hand get only with most (not surprising).
If your two armies rate as Battle Brothers, you share advantages, if you're more uneasy allies, then your two armies can freeze if they get to close (they don't trust one another).
Some things make sense - Space Marines for instance get on as Battle Brothers with most other chapters, the Imperial Guard and even Tau. But with Chaos? No way!
It's surprising how many other races the Necrons and Chaos can ally with. Seriously? Though they don't tend to make good allies.
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This seems a lot of fun. I have a pretty diverse Space Marine army in my Storm Wolves. I have large numbers of Terminators, Heavy Armour, Marines, Scouts, Bikers. I've never played the same army twice, and love the diversity.
My son with Imperial Guard and Tau is aware of their weaknesses. Now instead of a squad of Storm Troopers, he can throw in a squad of Space Marines instead and give himself a bit of muscle.
Likewise a Tau army without psykers can throw in a squad of Marines with a Librarian - that'll give you some psychic advantages, and a squad that'll fare well under close combat.
It's amazing who can be allies given the right circumstances ...
