Depends on your light and nutrients. If you have ideal conditions then Lilaeopsis brasiliensis is attractive, it needs plenty of light and no shade and propagates by sending out side shoots. A less demanding plant is Lilaeopsis mauritiana but you will have to propagate by splitting the rhizome. Blyxa japonica is beautiful but requires lots of light and plenty of nutrients as does Glossostigma elatinoides. Other carpet forming plants include Sagittaria sp. these resemble Echinodorus tennulus and are relatively undemanding.
I assume by mini grass you mean Echinodorus tennulus and micro grass Eleocharis sp., these are probably the least demanding and therefore the easiest to source. I've had success with Eleocharis upto the point where I'm now thinning out and discarding excess plants and replanting the stronger plants with tweezers.
Unfortunately you will to wait for a while (weeks or months) for all these plants to form a good carpet, your other option is to plants loads of single plants near one another but not touching and wait for them to propagate.
Hope this helps.