I think those rocks are really neat, and will make a nice scape. I'd get some kind of moss to grow over their surfaces in clumps. Fissisens would look really nice. You could also get some kind of groundcover, glossostigma is very easy and would grow well in that sand. I see that you were considering frogbit, but I might go with something like broad leafed water sprite. As for shrimp selection, tigers would look really good in there, and maybe some green neos. Looks good so far!