I would honestly limit your fish to what you have now. A firefish would work, but it sounds a bit cramped already. Yes, a clam can go in a 24 gallon tank, provided it is mature and has the appropriate lighting. The best species, though not necessarily the easiest among the clams would be T. squamosa, T. maxima, and T. crocea, since they remain much smaller than the other species (T. crocea being the smallest). Of course, if you plan on upgrading relatively soon, then you could try T. derasa, but this species still gets quite large.