Those gobies would work fine with clownfish.. I have that same one in my 75G. They are pretty aggressive/territorial fish for a goby, but only towards other gobies. So if you have a 29G tank that would probably be the only goby you would want to put in it... but you can put in a blenny as well, lots of those to choose from.
For cheap sand, I agree with Greech and recommend to play it safe and get caribsea dry sand.. you know it is good and not that expensive. Others try and go super cheap and get playground sand from a hardware store, but you have to be very careful to get the right type (calcium based, NOT silicate based) and that can be hard to find depending where you live.