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I have a notion on why perhaps the devloper can't add new types of turrets.
I would propose that the game already severly stresses the computer it runs on(As you may know, each server in this game runs on a single computer). Part of this stress is the need for the game to track the players with the turrets...Hence the limit on the number of possible turrets on a colony and their extreamly simplistic tracking method.
I would venture a guess that toonces CAN'T add new turrets because he would have to add new AI tracking mechanisms which would kill the already overworked servers?