I have kept ottos and discus together in planted tanks. Has always made me a little nervous, but have not found a good alternative for the ottos in planted setups. I have never had a problem. Ottos just can't be beat for keeping the plants clean, while not being a risk to eat them. Ottos are also one of the few species that really eat brown algae. I've never really had a problem with it, but still something to consider (especially in newer setups). BTW, I know plecos will eat it too, but they may also eat your plants and are deffinatly a risk to the discus.