98% of the time a midas or trimac will not grow any bigger than a foot long.
Hardly surprising when they're kept in 75g tanks....
Your logic of waiting for a fish to outgrow the tank seems incredibly backwards to me, of course you'll never see the full potential of your fish if they're kept in cramped quarters.
OP why not get something that can live comfortably in a 75g rather than one big fish that will "only just" fit? There are plenty of great CA/SA cichlids in the 6-8-10" range that could live comfortably and behave naturally in a nicely aquascaped 75g tank.