I vote similis or multi. I would keep them if I could find them for a reasonable price. I've heard that once a colony gets going they can become very aggressive to other species (part of the colonial behavior), so a species tank would probably be best. I couldn't tell from your post, did you keep the two species together or in seperate tanks? I've heard shellies hybridize readily, but I would think that buying established pairs would eliminate that behavior (at least in the parent generation). Just curious about your experience.