AwakenedRage

 If I am understanding this correctly, you are having issues with excessive amounts of Slime spawning in a certain area. The area in question was formerly part of the ocean and goes down into the ground significantly (Within the threshold that Slime Spawn).  This is the intended way that Minecraft is programmed to behave due to the area in question having more places that it can spawn then it would normally in a cave.

I understand that you want to disallow mob spawning in that area. The plugin you linked is more or less a replacement for WorldGuard and it isnt really that reasonable to add that plugin when it can be done within WorldGuard. I can hop on the server later and disallow Slimes spawning in the area in question if you want. 

Would you like that or am I missing something?

Regards,

Michael