Not 100% necessary in small tanks, but it's the best light for them especially in the long term. Clams without MH are a gamble. I've seen them do just fine, and then again, in a fine tank without MH, I've seen them perish after about 6 months do to no other known factor than lack of lighting.
If I were you I would try a derasa clam, or perhaps a squamosa. I'd steer clear of maxima or crocea....they're too big of a gamble, IME.
If you're buying new light, why not MH? You can keep anything lightwise, unlike others, and the price is about the same in the end.