I have a 10gal. tank, a 5" Kissing fish, a 6" upside-down catfish, and a 6" algae eater...and a mystery...?

No real way to tell who it is... but 2 barbs is not big enough for a school and 10g is not big enough for all those fish. You're probably experiencing territorial issues because the tank is too small, or the barbs nipping at each other because the school is too small.