I ditched my hornwort because of its reputation to drop needles and make a mess.
In my RCS tank I keep wisteria, ludwigia, java moss, najas, hygro, dwarf sag, vals, etc..
The things that provide the best cover are java moss (for shrimplets) and wisteria which grows like mad. Honestly the shrimp love it all. I think I would also recommend dwarf sag.. still early on mine, but I think it'll grow in for some good low cover.