I've not mismatched RAM for a long time. It is supposed to gracefully degrade, and everything run at the speed of the slowest. I assume that still works, but don't have any firsthand experience.
Is the price good enough that you could go back and buy 3 more if you do end up with compatibility issues?
Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs