You have no chance of the plec droppings working their way through the gravel and fertilising your plants. You will need to do lots of gravel vac'ing. I have a small (2") sailfin and I am cleaning the substrate in my tank every other day. If you don't keep on top of it you will end up with horrendous water conditions. in a 20gal tank you'll be looking at 20-33% water changes every 2 - 3 days.
I think as well that whilst 1 might be ok in a 20gal, 2 is pushing it. What filters are you running and you need to make sure you're getting a good amount of oxygen into the water.
Also, plecs aren't actually the best algae eaters even though that is what they are often sold as. Mine eats cucumber, Hikari algae wafers and some pellets I bought off ebay (I'm in the UK and they're a shop own brand so you probably can't get them over the pond).
Seriously, from what you've said so far I'd take them back. I'm not having a go at you but unless you can commit to that kind of tank maintenance it will be better for you and better for the plecs.
Cheers
Danny B