I had a small tank for a year, it was about 24lx18hx12w and I had one 65w pc 50/50 and I had no problems growing anything in there (never tried clams though).
In my experience, you should try your suggested set up and see.
Everyone has their biased opinions based on their own experiences, the topic of lighting is one that many take personally for some reason and you will find that every opinion differs at least slightly.
Trial and error is the best way to go, set up your system the way you would like to and then see how it works, move sps around, play with the positioning of your lights over the tank, etc.
you can always adjust and upgrade and then worry about what others say will and will not work after you've attempted your own hypothesis.
I think your set up would work fine, though you may not be able to have some organisms that require higher light at the bottom of your tank, of course this is only my opinion based on my own experiences.
cheers.
