We have HC growing suprisingly fast in our 75 gallon with moderate light, diy CO2 and a nano diffuser. It might not be ideal conditions, and it might not be growing superfast, but it's doing a lot better than we expected it would.
Another plant i've found good for carpets is UG it's growing like stink in the above tank and we have SUPER hard water. So is the Dwarf Sag, but it's quite a bit larger and won't give the same lush carpet effect.
Dwarf hairgrass on the other hand, didn't work out fantastically, it grew, but really slowly. We've since moved it to another tank where it gets more light. Strangely, it's stopped growing all together there.
Basically, i think you just have to find a what works for your tank and your water conditions. I mean, we've got plants growing in conditions that people said would be impossible and apparently Easy plants that are dying off. Try something for a couple of months, if it doesn't work, try something else. Eventually you will find what works for you.