Ottis aren't much for thread type algaes. The SAEs love the stuff. Yeah, they get a bit big for a 20 and they get big fast. I have 3 in a 30. As for belligerent... I don't know. I have them in with threadfin rainbows and glass catfish. You would think there would be an issue with the threads if they were bad. Only thing I have ever seen is a large chunk of my grandpa glass cat's tail missing. I have noted he no longer hangs out in the cave with the SAEs. I have my suspicions but no proof. They do an amazing job on the plants though. Maybe a little too good. I think they have stripped every leaf from my hydrocotle. The nerites that breed in salt water reportedly have a serious thing for algae, and don't eat plants (or breed in fresh water), hence the mention of them as an option.
UV makes a good adjunct, but a poor primary treatment for existing algae. It will help prevent it, and prevent the spread. It's good for green water, but by itself it won't get rid of this.