I would not put four breeding pairs in a 75gal., especially with one of those pairs being Convicts. They will breed more regularly than the others and become very territorial. If you happen to get more than one pair to breed at once, then more problems will occur. A 75 really isn't a big tank when it comes to keeping multiple pairs of breeding cichlids. You could try two pairs of the smaller species or one pair of a larger type. If you are trying to stock all of these fish because you want more movement, get some dithers instead like danios, barbs, or mollies.