The thing with Beacon is they are rigid in all but the most dire of circumstances. Even the agreement with Penny and Laat is temporary and would not likely be extended to other monster girls. Penny so happens to be more of a technological amalgamation which falls in a workable area for the moment. All the others thus far are the more traditional monster type which the Beacon have been trying to extinguish for generations. Monster girls are seen as corrupted and non-redeemable.