you could try cryptocoryne plants for a foreground . . . they have decent root systems, and they will send out runners as they grow, so that their coverage gradually expands . . . and they do fine in low-light, low-tech set-ups
I saw the reference to bichirs, which I don't keep, but I do keep cichlids and bushynose plecos, both of which can be disruptive, but eventually the crypts will take hold
you can also try planting the crypts in pots . . . they will hold better, and still send out the runners . . .