Considering most of the rules are done from a "fairness" idea or a community agreement, you would be hard pressed to find someone tell you to no run bloodletters with your world eaters. Chaos Daemons and Chaos Space Marines are a pair up that are pretty much universally accepted seeing as we never got over the whole codex split.
Now for my next trick, where it would be acceptable to run 3 codecies together!
Lost and the Damned themed armies are pretty much a staple in the 40k universe but GW found it prudent to remove them. Running a renegade guard (Imperial Guard codex) with a splash of Chaos Space Marines and Chaos Daemons would also be something that would be ok considering how they tend to run together.
Considering the above listed example was once an army (a personal example), you would be hard pressed to find someone even frowning on such a list because of the care going into it.
At the end of the day it's about bringing a cohesive force that you put a lot of effort. The more love and effort you put into a list, the more it will be accepted.