whats the reason for putting the legs on
could i add hairgrass
and how long will it take the micro sword to carpet i know i put pygmy sword but i was wrong
Let me try to answer all of these one at a time as best I can.
You want to have the legs on your lighting so for what ever reason if the top comes off or breaks the light will not fall in and fry everything in the tank and have the city come out to fix your breaker ;p Also it helps with not have in 'right on the glass' because of heat and evaporation. Some people might vary in this opinion.
As for the Hair-grass. With your setup it would do fine, but I noticed that you have no CO2 going. With the crazy amount of lighting you have, you really should think about getting some CO2 going. On the Flip of this, hair-grass is extremely resilient and can hold up to almost anything next to the tank running out of water or it getting eaten. A lot of people use it for carpeting but you can use it as an accent as long as you would keep an eye on it.
For how long it would take to carpet the tank with a micro sword, or the likes of the same kind of plant, that really depends on if you are going to start it off emerssed, and if you are going to add frets and/or CO2. It's really hard to put a finger on how long it would take, and for your size of a tank, but any carpet take a while(more than a month or two, ect). Sorry I can't give you a better answer with that. You have to think about it growing in a way you don't want it to, perhaps it will bush up rather than out. There are just so many things.
Hope this answers your questions or at least will get you thinking in the right path.
Also nice start you have going here. And +1 for the sand river ;p