I have my doubts about that fixture fitting a 10g. 39w T5 HO bulbs go in 36" fixtures, in other words you have 16" of excess fixture hanging off of the tank and you will lose a lot of light. Don't go by WPG anymore as it is outdated and factors such as PAR, intensity, and penetration are more important. By 3 of each in the 30g, 3 cardinals would work perfectly, but 3 clowns would be a disaster due to agression/territorial issues. In the 10g, I would say no more than 2-3 carefully selected fish. Four pounds of live rock per gallon is probably WAY too much unless the rock is really dense, but it would still most likely be too much. I agree with Zero, look into gobies for that 10g.