Think of Killis as Betas. Males will atack each other. In a 30 gallon you can maybe put two males. They like to have maybe two females per male. Be aware that when mating, males can do serious damage to females. In a tank that size maybe would be better adding 2 different species of Killis, but if you do so, try them to be not from the same family to not damaging the genes
Important things to remember before selecting the Killis:
1. They JUMP. Cover EVERYTHING, even the filter as much as possible
2. Some swim on the top almost always
3. They are carnivorous, so flake only diet with hurt their colors
I doubt you would have any trouble keeping them, the only trouble is finding them. Im about to meet a breeder here, but he lives 3.5 hours from here and because the school year is ending Im having trouble finding time to go there, as I will loose an entire day. Im looking for info about how well they do when you mail them. So if you cannt find them, let me know. But, it might take some time. Good luck.