I would suggest a bunch of hatchet fish, a few shoals of smaller tetras, a group of discus, a group of smaller geo's, and maybe uaru! That would be my ideal setup.
I agree black sand will absorb light but some fish actually show better color with darker substrate. Depending on the large fish you choose you may have to becareful of the small fish as they may harass the large ones. I know our giant red tail gouramis would'nt eat the little convicts in the tank as planned. If you want pure entertainment get some of the bigger pike and fill the rest of the tank with small inexpensive cichlids and watch the pike hunt if your are into that kind of thing.
It depends on what size your bigger fish will be, if your thinking cichlid theme then you could add shell dwellers, mbuna, and peacock chichlids all of which like to hide in caves and rockwork and are just the right size not to be eaten (but depends entirely on which large fish you have obviously)
Try zoosand.com or something like that. I'm thinking about seeing if they will send a few samples they carry it in like 450lbs and it's pretty reasonable. Or 20-20lb bags of Tahitian moon lol