Honestly, if Shadow is going to be a standalone type Pokemon either are or are not, Shadow walls will be OP, not Shadow attackers, since absolutely nothing will do better than neutral on them. Attackers would much rather have the ability to be super effective, and Shadow attackers are hard-walled by Shadow walls (Or Shadow Dry Skin Pokemon, for that matter), making them a bit lame. Eviolite Shadow Chansey is going to be hell to kill.
I wouldn't worry too much about capping them or whatever until the mod is actually coded, though.
How are players going to select Shadow? Shininess?
Is there going to be any representation of Hyper Mode/Reverse Mode? For reference, both of them cause the Pokemon to randomly disobey orders to use non-Shadow moves (In some interesting ways at that, such as returning to their Pokeball spontaneously), they both block you from using Items on the Pokemon (Auto-Klutz?), Hyper Mode increases the Critical Hit Ratio of Shadow Rush, while Reverse Mode causes the Pokemon to lose health at the end of every turn. Also apparently Pokemon outside of Reverse Mode are prone to disobeying an order to use a Shadow move.