hanif, I would go with the white sands of Dover, but, failing that (and hoping that you realize that I was there kidding), any coralline sand safe for FW aquarium use would be a good idea. Ocellatus will be stressed without the ability to bury their shells (and selves at some point) and, as Kit Walker stated, pea gravel does not allow for that. I don't see rock being all that necessary as you already have good buffering with the sand and possibly within your available water supply.
I think that you could stock more in that tank, but with 1m/4f, you are looking at starting a long-term breeding colony, I assume, and you will be able to make bank (or at least a few pounds) out of the deal if that is the route which you choose.
Once you get this tank set up and going, it would be nice to see some pictures!