In that small a tank I could suggest things like Dwarf Anubias, Wendelov Java Fern, Mosses (one of the more compact ones, not the stringy Java Moss). Crypts are likely to stay smaller under low light conditions, so if that is the way you are thinking, go for it.
The lowest light ground cover plant is probably dwarf Clover, Marsilea.
I totally agree that the dwarf clover is definitely a great plant for you. I also think that the Pygmy Chain Sword will definitely be good for you too; however, these kind of bush up rather than make lush carpets...Do check into it, I heard it grows realtively quickly even in conditions without co2 and high light as this plant doesn't like high light anyway.