I kept four pairs of different killifish at one time.
Pros: beautiful, easy to keep, many are easy to breed, unique.
Cons: I didn't like keeping up with four smaller tanks in addition to my two larger tanks, males can bully females if kept in pairs only, some will rip fins of other fish if kept in a community tank.
Overall, I really enjoyed them and would recommend them. I did my research, and much of what I read said they could be kept in pairs in tanks as small as 2.5 gallons with no filtration if water is changed regularly.
However, I think they would do best in larger tanks with filtration (makes maintenance easier) and lots of cover for the females. I think it'd be best to have more than one female per male; lone females can get beat up.
Good luck.