I think what you're looking for is in Mob.cs Ln 753, or you can just search Respawn(); in mob.cs. And what you're looking at is
Code:
RandomTime = (uint) (Rnd.next(20, 60));
So you have to change that interval 20,60 which is how soon they spawn in seconds so if you want them to spawn instantly change it to (1, 2) or something like that. I haven't tested this but I'm pretty sure thats where you change it.