There is risk, anytime you mix other fish with bettas . In your case risk would be minimal as your betta is already living with other fish and i wouldnt expect your betta and otos to live in different parts of the tank. Always a risk, but bettas and otos are considered good tank mates.
Ive got 4 otos in a 15g tank and will be introducing a betta in there shortly, with a spare tank on standby should it not work out.
No direct experience of using Fenbendazole with otos, but a bit of reading around shows people do treat otos with it and use in quarantine too. Thats not a personal recommendation though.