My general experience of riccia so far is that you really do need a lot of light and some extra CO2 to get it to branch out and grow a lot.
How ever, once you get it grown to a point that you're happy with the coverage, you can lay off the CO2, or even have a bit less light and it's pretty stable. It stays alive, but new growth slows appreciably.
You definitely do need to tether it down too. I'm not sure how you'd best manage that to carpet the substrate, but to cover bog wood or river rock, wrap a layer of it under tulle netting. Eventually it will grow through the net so much that you won't even see it.
Right now I'm trying to grow out some riccia on river rock in my bright little quarantine tank with CO2. When they're as big as I want them, I'll move them where I want them and bury the bottom of the rock in the substrate, so all you'll see is the riccia.