But if someone has 30 zombie pigman sitting in one 16x16xbedrock area. (1 chunk), it doesn't take away from an overall pool of available mobs. The reason I say this is because despite hostile mobs spawning, you can still breed passive animals. If the "cap" was it, it would not be letting in new passive mobs either, or it would be removing other mobs within the world. We're not receiving enough complaints of people losing their animals to really justify that explanation.
I suspect the issue is Skyblock is broken, the entire plugin, utterly broken. There is something interfering with how hostile mobs spawn, finding what that is and fixing it is the solution, really.