the mh is fine as long as you have it at least 8" above the tank. You might have some heat issues.
As far as clams are concerned, most don't grow so fast that it will outgrow your tank anytime soon. The Derasa's, Squamosa's, gigas grow faster than the Maxima's and Crocea's, but they are easier to keep. I personally like the Maxima's and Croceas. good luck.