Definitely MH first.
Second: Best bet would be to get a clam minimum 3"-3.5", as clams that are smaller than that require a higher percentage of their nutrients from feeding rather than photosynthesis. As they get bigger they rely more on light.
If you are going to put a clam in a smaller system like yours, you'll have a much better survival / growth rate with a clam that is "out" of that infant stage. You'd have to either pull the little guy out for feedings or risk really overdosing the hell out of your small volume tank.
Also, be prepared to dose kalk: these guys do a number on calcium levels in small tanks. A good idea for topoff anyway with halides heating things up, too.