Mollies don't mind hard water, even prefer it, so a tiny bit of fizzing is not necessarily going to cause problems in your tank. It depends a whole lot on the water parameters in the tank already. If your tap water is moderately hard and alkaline, adding slightly calciferous rock is not going to impact things much at all. If you have extremely soft, acidic water then it will be more likely to alter the chemistry.
I'd be more concerned about heavy metals within the rock, like rust colored streaks, that can be toxic to fish.