Color is easy in this regard: E-6 is E-6 is E-6. C-41 is C-41 is C-41. It is black and white that is the problem when it comes to mixing films in the same processing batch! Color films are designed to work in a standardized process. Black and white films are not.
Of course when you start messing with non standard processes such as cross-processing E6 in C41 or vice versa, all bets are off
"Flatter Me, and I May Not Believe You. Criticize Me, and I May Not like You. Ignore Me, and I May Not Forgive You. Encourage Me, and I Will Not Forget You."