Should work OK. For bright colored acros, the 250watt pendant would no doubt be much better. With the 20k, this can be your only light source. The 20k AB blub is a very good bulb. I use it myself. Just a side note, I've noticed some online vendors switch from the AB bulb that Aqua Medic supplies with the pendant, to a cheaper XM bulb. Shop carefully.
I would think clams would do well with that. I have had good luck with T. deresa clams under PC light in a shallow tank, and under 175w MH, but at the bottom of a deep tank.
I would think that's enough for a maxima clam too, especially up on the rockwork, but correct me if I am wrong. They need more light than the deresa clams, but I don't think they need an overwhelming amount.
But you could surely put a deresa clam under that. Be sure to keep up with calcium!