I don't know much about SW but, i'll try
I don't know about the pair of clowns you might want to narrow it down to one............here is how i see the stocking level is LxW then divide by 72 and that is how many inces youi can put in there..........but, if the clowns are small and you plan to get them to a bigger tank i guess it would be okay
I guess the sand would work only if you clean it quite a bit.............
Filtration sounds good but, you might want a power filter or a power head in there for movement.
IT sounds pretty good so go for it
P.S U might want to Ask OBK for more Info
Best of luck,
Daniel