I'd think you'd be fine for awhile. I have asked this same question too in figuring out a tanks potential and a lot of guys have just told me to try it cause it is so situational from fish to fish. Good luck man!
I think you could do that in there as long as you watch the behavior between the fish. I have a 180 gallon that is way or stock and I have some big fish in there and just add two more fish so yes it is possible but in the long run upgrade the tank or split them off into to tanks then. Just my 2cents
I would say try it out. You seem to understand the risk involved. There are more ridiculous stocks out there that have worked out. I think in the very long run it just may be too crowded, but that is a good amount of time to build a sweet pond! Or save up for a bigger tank.
It all depends on the personalities. Are they all going to be young fish? That might help for a while at least. Just keep an eye on the aggression. It is good you have the 75g. I wouldn't put any cichilds in there until you find an over aggressive fish, which the chances are good you will have one. It is always good to have a back up tank.