I think maybe 3 Badis badis would be ok. I have no experience with them, and I am speculating based on their size.
For the shrimp, you could have a lot without doing much to your bio load. The shrimp might become a snack for the Badis badis though.
I would suggest against a clam. They are boring and all they do is bury themselves in the sand. You also need a very well established tank, and maybe a little dirty, but without ammonia issues. They are hard to take care of. Very few people successfully keep clams long term. Clams don't actually have brains, so they don't do anything at all. They seem to know to bury themselves though.