And doesnt the lab eat the demasonis babies and demasonies eat the labs babies?
I highly doubt it, but if that is a serious concern, you could always just stick with one species (ie, yellow lab colony, demasoni colony, Saulosi colony, bumblebee colony, etc.).
Also, you could do a really cool Tanganyikan setup, there are plenty of Tanganyikan cichlids that would easily fit in a 75, heck, you could get many of them in 40 gallons above. There is like Julidochromis, Fairy cichlids, compressiceps, etc. A few people have some shell dweller setups among others, I forgot who, but maybe they could comment. The only downside to these types of African cichlids is that there isn't a whole lot of color, a few have color. But they make up for the lack of color with beautiful patterns. So if you are looking for color, I would go with an Mbuna colony or possibly some Peacocks. Peacocks are smaller than Haps but still give a ton of color (admittedly, probably more color than your average Hap). If you don't mind less color but enjoy patterns, Tangs are your best bet.