So would a 6ft x 3ft x 2ft be okay for the mix of those mentioned cichlids ?The difference between a 300 gal, and 200 gal is significant.
I have seen fish kill each other in 150-200 gal tanks, that live very quite comfortably together, or at least maintain a sense of "detente" in 300-350 gal tanks.
There is something about that extra space that makes huge difference.
If all put together when young, at the same time, without any stock changes along the way, you can try....maybe you will be an exception to the norm.