In my 55 gallon community tank there are 3 buenos aires tetras (that came with the tank) Everything I've read about them says that they love to eat plants, so I've been a little scared to start planting my tank.

Reading about Anubias Nana, I figured that it had a tough reputation as a bitter plant, so I picked one up and put it in my tank... one by one it's leaves are being munched into lunch >_<

Could I move these three into my 20 gallon tank with 5 black skirt tetras, for the sake of my plants?

I have had Buenos Aires Tetras before and have never noticed them to be plant eaters. Two plants you might try are Vallsinaria and Java Fern. Both are very tough and not normally eaten by fish. Remember that if a fish is eating plants, it needs them in its diet. Try providing them with Algae tablets as an alternative.

I do give blanched zucchini and algae tablets on alternating days.

The java ferns have been doing great, though, as well as the amazon sword