Like spookytooth said, cories do best in 6+...the smallest I would put anything like Pygmy cories in is a ten gallon...MINIMUM.
Some good low tech plants are java fern (Wuxi I already see you have...tie it to driftwood or something, don't burry it), anubias (EXTREMELY durable...never been able to kill one yet! But it's another plant that needs to be ties to something or left floating), crypts are good, swords should worm for you, java moss, ludwigia I think, elodea (or anarchis...but it tends to be a bit messy...another good floating plant), hornwart (good floating plant), frog bit (floating plant)... I know you asked for ones that go in the ground, but I love floating plants...love the look, so I mentioned some of those, too
The plants that aren't floating plants that I mentioned should do fine in the sand. Mine do, anyway...also I think that you could stick hornwart and elodea (anarchis) into the substrate if you wanted to, but I think they're hard to get to stay down, and they look good as floating plants...and many bettas love floating plants