I think an opaline gourami is going to get a bit too big for 20 gals..plus he may pick on such smaller tank mates. Maybe a neon blue or sunset dwarf gourami would be a better fit and add a nice splash of color. A common pleco is pretty big for your tank and has a big bio-load. I think a hillstream loach (commonly sold as a butterfly pleco) is a better algea eater for a smaller tank...so much more attractive and stays very small, only a couple inches, versus a common pleco that gets huuuuge. Just my opinion
Cories are super social and their cute behaviors really don't come out unless they have a buddy. Personally I wouldn't keep less than 4 but depending on the adult size of the type of cory you have you may not be able to house that many so I think 2 at the absolute minimum of the same species if you can.