Is there any prohibition against using or firing speckled clay ie Laguna speckled buff?
Nope!
You should probably consider making yourself some non-speckled “cookies” to go under your pots. The speckles stick a tiny bit to the shelves, and it’d be nice if there was a cookie between the pot and the shelf. You should also make some “feet”. I make clay balls, and then flatten them. If your glaze runs unexpectedly, the extra adhesion of the speckles can make it very difficult to get your piece off the cookie.