You can basically keep any plants you like that will grow under your tank conditions (lighting, substrate, etc). Some good floating ones are duckweed, frogbit and hornwort. My girls love the dense cover that duckweed provides (I hate it, but eh, it makes them happy, especially since my tank is open-topped).
With a sorority of say, 6, 2 guppies (I'd get another one, as they like groups of 3+), a school of 6 tetras and 6 cories, you still definitely have room for a mystery snail or nerite snail if you want one. :) For a twenty gallon, my general rule is: 2 schools of mid-top swimmers, 1 school of bottom feeders and a centrepiece fish. You can substitute a snail for the centrepiece fish without any bioload dramas, especially with live plants. :) I know yours isn't a twenty gallon, but I'm allowing the remaining 5 gallons for the guppies.