I have a 20 gal
and so far there is 15lbs of rocks in there, but I plan on getting more and figure it will be closer to 30-40 lbs by the time I am done. In the past, there were cichlids in the 29 gal
with slabs of basalt and Texas Holey Rock.
I wouldn't worry about the glass bottom. As long as the tank is made properly and you are not dropping the rock in and letting it smash the bottom, any rock you add will be fine. I will add that the tank needs to be on a stand intended to hold aquariums to provide the best support.