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Here is my theory on how players can take chaos space marines AND take imperial knights in the same force and have it perfectly legal....

The Imperial knights codex says it is a special type of detachment that does not take up the option for the primary detachment to bring in an allied detachment. So this force could in theory work

Primary detachment = imperial guard
Allied detachment = Chaos Marines
+ imperial knight detachment

Since both imperial knights AND chaos space marines are able to ally with guard and imperial knights do not stop the guard from being able to ally this sort of list would be legal.

Can anybody quote the rule that would stop this?