Allies

[UltraTim]

I don't understand how it works. can they only be used with Demon hunters? i could i, for example, mix orks with SM

[WarpSpawn]

In vanilla 40K (not Apocalypse then, though not sure about the rules for that), there are no general rules for allies. If you are playing with more than one player on each side, you decide amongst yourselves who plays on which side and with what. Anything goes in this situation, as long as you all agree to it.

 

For a standard 2 player game, normal practice is to choose a single FoC from a single book. Anything beyond this is between you and your opponent but I would not expect it to happen much in organised competition.

 

Daemon hunters and Witch hunters are exceptions to this. They can ally with people because their codices say they can. You stick to the rules found in those books if you want to go down that route.

[Captain Malachi]

To put it bluntly, no. You cannot ally orks with SM unless your mates let you as WarpSpawn says.

[Gurth]

Daemon hunters and Witch hunters are exceptions to this.

As are Kroot mercenaries, but they can't ally with Space Marines. Other than that little snag, if you want to field them you would simply use the rules given in their army list as well.