You could give normal weapon platform guns the Engine ability. Have normal WPs have a high max engines, so that they can have as many weapons as will fit. Then, the Gropos WP would have 0 max engines. It would only be able to have the huge mega cannon thingy on it, and would not be allowed to have normal weapons on it. Of course, this requires that all of the weapons placable on WPs have to a different entry than the ship Versions of them, which might be too much work.
