Chinese Algea Eaters are the main culprets when it comes to sucking onto and attacking other fish. As stated bristle nose catfish have been known to do it too on occasion but mainly only on discus (in my opion this may mainly happen when BN's aren't getting enough protien in their diet) and once they start this habit its all but impossible to stop. Otocinclus are usually innofensive but have on the odd occasion been caught snacking on a discus's slime coat. Generally however otocinclus are the most vegetarian of the sucker catfish. The ones that have been caught feeding on fish may have been a particular type of oto, or had other reasons eg. needing protien in order to successfully bred or missing some mineral in their diet.
Generally though otocinclus are a safe bet with almost all none aggressive fish.