I could be wrong but I think the dwarf frogs wouldn't like the sharp gravel either, and I know the kuhli would not. Any chance of changing the substrate to sand? It would not be expensive to change the substrate in a 10g.

Oto's are not schooling fish, but they are social fish. They need company, but smaller numbers are ok. In their natural habitat they group in the thousands, in our small tanks a few is fine.