I am toward the end of an RF transceiver project quite like this. It has a military application, so I am not able to share anything with you.

If the number of modules "in field" varies, and you are not doing so already, you might consider having the primary station (master) broadcast a "ping" request to the secondary stations, which will then turn around and emit a short "ping" response after a random delay. The primary can get a rough count (rough, since collisions are likely to occur) of the secondary stations it's dealing with and then tune those parameters accordingly. The primary then broadcasts the parameters to the secondaries as part of the protocol. Oh, and the "ping" request could contain the size of the randomization window.