You'll be fine, even a full grown oscar wouldn't be able to choke down a 20cm (8") pleco. I've got a 8" comman in with my O's (ranging from 4"-13.5") and they have never attacked the pleco.
Thanks for the advice all. I've included a large mangrove root for the pleco to hide behind and he doesn't seem to be stressed out. Hoping for a happy tank.
Saskia
