Idk if its possible, but having mobds only in the randomly generated in the dungeons and caves it minecraft would be great, and work well in my server.
Try changing Code: spawn-animals=true to Code: spawn-animals=false in your server.properties. It is already built in!
A really good workaround for this (that I use on my own server) is to use the Noon plugin. Yes, the sun never goes down, and that's how you only ever get mobs in caves and dungeons, but if you don't want danger overground, why do you want it to get dark, either? My guess is, if you do all your building overground, and you're asking questions like this, using the Noon plugin will solve all your problems. Unless you specifically, aesthetically want it to get dark sometimes.