Males tend to have more speckling near/on the cadual fin area. Depending on the race(locale) they may also have a deeper background color to them. Females tend to be more round in the belly. note these are all fish that are at least 3inches before that I have no clue how to tell. These are great fish they are not belligerent but will put up a fight. I kept my last group with some Amphilophus Rhytismas & neolamp. leluepi the splash of color was mystifying. You have to breed them if you can they can do ok in a 40 or 55 gal. with some hide spaces.
How big & how many an adult pr. will just squeeze in with a small cat for clean up 3 or 4, 2" -3 " will be ok for awhile. These are semi aggressive so may be ok if you have good filtration as they are sensitive to water quality.