This way, you can have several completely different types of unit spawns.
The maximum manpower cap set in the Spawn AI module applies to all synchronized Spawnpoints. Set the weighted ratio of each module - infantry, motorized, mechanized or armor - and synchronize it with the Spawn AI module.
To stop the hostile AI units entering would require editing the underlying pathing so that they will avoid it (and thats very hard to do). This is because of the way that pathing works. It stops units spawning inside it, but it doesnt prevent them from entering.