What you’re trying to build is called a Wireless Sensor Network (WSN), and Bluetooth is not designed for that – and even if it were, there are smaller protocols that are lighter-weight and easier to work with.
The XBee modules are used almost universally by the hobbyist market for the sort of stuff you’re trying to do. Series 1 modules are basically “wireless serial” links, which won’t be very useful to you, while Series 2 devices offer more useful mesh networking support (including support for access points, routers, and end devices).
An XBee Series 2 set-up would be perfect for what you’re trying to achieve – you’d have your “master” hooked up with a S2 XBee programmed as an AP. All of your slave devices would be interfaced with S2 XBees programmed as end devices. Once they associated with the master, the master would be able to send messages to them individually, or to a broadcast address that they could all listen to. The XBee device takes care of the MAC and ZigBee stack, so you don’t need to worry about implementing that.