All good comments/ideas. I would add that hairgrass (IME) needs lots of CO2 to grow dense enough to create the carpet effect. My 46gal (click in the signature) is covered with 70-80% hairgrass.
Microsword is one of the slowest to form a carpet. I would stick with hairgrass, glosso, or riccia. Dwarf sag can be nice too, but doesn't have the same look as a "normal" carpet. It's kinda "flowery" and height can vary. And remember, riccia floats
FISA brings up a good point, hairgrass likes iron - eco complete might be a better option if you can afford it.
Regardless of the carpet plant you choose, I would recommend putting some fast growing stems in the tank at setup. Algae blooms can really set back, or even kill off, carpet plants.
Buy 2 or 3 pots. Get yourself a pair of forceps. Separate the hairgrass into individual plants. Roll up your sleeves and set aside 1/2 hour. Spread evenly thoughout the area you want to carpet. Unlike some other grasses, you can push hairgrass faily deep, so don't sweat about suffocating it. The runners will get around.