Agreed they need a group a mix of males and females is best. Rainbows do breed very easily however mine have never shown any aggression to any other fish apart from rainbows. Mine spawn and completely ignore their tankmates eating their eggs. I have two males and 4 females only aggression is chasing between the males. The eggs don't stand much of any chance in a community tank while they are easy to spawn they are not that easy to raise since fry are very very tiny and usually require specialized foods.
They are very peaceful community fish IMO though they are very active. Also of importance is they are very very food driven fish, they are not food aggressive but they can out compete more docile/shy/slower fish at feeding time. So make sure all fish are getting food and in a 55 gallon best option is to spread food out lenght of the tank. They are actually more likely to show aggression to other fish when kept alone. They are naturally social fish which forms a hierarchy in a group, without the group or with too small of one the fish may try to do this with other species outside the norm. There are always dominant fish in a group of rainbows, typical rainbow behavior always involves daily fin flaring and some dancing between mature fish to maintain their social order mostly amongst males. They rarely ever cause physical harm or go beyond posturing unless a fish tries to move up in 'status' or an established group is enlarged. In both cases there can be outbreaks of heightened aggression usually between specific fish until they figure out who exactly is boss after which aggression goes down. So do be surprised if you see some of this when you add more rainbows.
Far as your fish being a male or female, sexing rainbows is challenging and from the photos I can't tell. Males are usually more common then females in stores but this varies from species to species. Some species have very drab females and are easy to sex while others don't. I have never kept turquoise rainbows in particular. The best way to sex the larger rainbow species is to spend a good 15 minuets watching them in a shop tank especially if looking for females. All the Melanotaenia species (far as I know) follow the rule that a females first dorsal fin will not overlap the second one. This in my experience is the most accurate way to sex them but its also not easy to determine in a store tank. Going by color alone is tricky as these fish can change color extremely depending on mood and usually stay very pale in shop tanks. This
link shows a very good picture of a female turquoise.