The female will be the largest will the male next biggest. The rest should stay juveniles. Establishing an adult pair This is little tricky and a close eye needs to be kept on them to make sure that the female doesn't kill the male. Buying a large adult and getting two smaller ones from a group and letting the female pick one has been done with a good deal of success. With maroons try introducing just one male at a time and keep a close eye on them.
Fresh shrimp frozen and then grated is a good nutritional source.
Remove any aggressive fish that might make them feel threatened. This takes some preparation and is really beyond the scope of this page. I recommend reading the Plankton Culture Manuel from Florida Aqua Farms. Its tells all you need to know and more about raising nannochloropsis oculata, greenwater, and Brachionus sp., rotifers. They also can provide live cultures and starter kits.