keep a 3f 1m ratio, lots of caves and plants incase of bullying but leave some open sand space as they like to use this to spawn.
when picking out one, pick one that is female in appearance and behaviour, but stands its ground. introducing weak or smaller fish to an established tank means probable bullying of the new fish. also, take out the dominant fish/ 2 fish from the tank for a day and put it/ them in a breeder net for the new fish to acclimate and get some territory/ tank geography before being harrassed.
what do u already have in the tank, how big is it, what caves etc do u have? x