Connect multiple devices to one device via Bluetooth

Yes, that is possible. At its lowest level Bluetooth allows you to connect up to 7 devices to one master device. I have done this and it has worked well for me, but only on other platforms (linux) where I had lots of manual control - I've never tried that on Android and there are some possible complications so you will need to do some testing to be certain.

One of the issues is that you need the tablet to the master and Android doesn't give you any explicit control of this. It is likely that this won't be a problem because* the tablet will automatically become the master when you try to connect a second device to it, or* you will be able to control the master/slave roles by how you setup your socket connection

I will caution though that most apps using Bluetooth on mobile are not attempting many simultaneous connections and Bluetooth can be a bit fragile, e.g. what if two devices already have a Bluetooth connection for some other app - how might that affect the roles?

Bluetooth 4.0 Allows you in a Bluetooth piconet one master can communicate up to 7 active slaves, there can be some other devices up to 248 devices which sleeping.

Also you can use some slaves as bridge to participate with more devices.

Yes you can do so and I have created a library for the same.
This allows you to connect up-to four devices to the main server device creating different channels for each client and running interactions on different threads.
To use this library simple add compile com.mdg.androble:library:0.1.2 in dependency section of your build.gradle .