Rummynoses are my jam. I have 9 in a 20L (used to have 10, but one guy died on the way home from the store -sob- it was him and his buddy alone in this tank so I just took them home to add them to my school, SIP)
They show no inclination towards nipping anything but each other to sort out school politics and are, as far as I am concerned, really easy to take care of. I only had a betta with them a short time until his own tank cycled, but they got along great. When I first added the betta temporarily, they zoomed up to the bag to check him out and followed him around for the first few days to see what was up. They are A+ schoolers and they are very curious fish, mine are not shy in the slightest and they want to know EVERYTHING thats going on in the tank. So if you want a schooling fish with a lot of 'tude and a fantastic personality Rummynoses are amazing!