I'm not fully sure on your total stock right now but a swordtail, platies, and an algae eater (pleco I'm guessing) would put you at about the max for a 20 gal
. If you're looking for an entirely new dimension to aqua keeping I'd highly recommend trying out live plants. There are quite a few low light low maintenance types that can do perfectly well at minimal cost. Once you go live you'll never buy a fake plant again!
Honestly plants becomes practically more fun than the fish themselves at times.
Edit: Thinking about it more if you rehomed the pleco and swordtail you could easily do a honey/dwarf gourami and/or a German blue/Bolivian ram. Just some colorful options for you, but I would not add them until your tank's stock is lighter.