Bluetooth is a radio, just like WiFi and there is a slight battery drain for each device used but is minor. It is very little and almost not noticeable but with these tablets, you have to set them up and then put them to use and monitor the battery drain as you go.
You also have a range of about 25+ feet as well with them. My VIZIO headset, I have travelled further and had perfect connection. You won't be doing that with a keyboard or mouse with a screen that small.
Do note that each of these tablets have a life of their own. You can read that here in the forum. Just because mine works that way, doesn't mean everybody else's tablets will work the same, so, you just have to try it out.
The device limit for Bluetooth, like I said, is not a set limit. It depends on what class of Bluetooth that was used for the tablets. Normally, most people don't have enough devices to fill up the full allocation. We are talking about Bluetooth and you can also communicate with a cellphone via Bluetooth as well to transfer files. You may find issues doing that with these tablets even though it is supposed to work.
Experimentation is about the only way to find out what these tablets can do. If you dig around on Microsoft's web site in the Surface category, look at the bottom of the web pages and there is a PDF booklet which is for the Surface RT and Surface Pro that is fairly good if you want documentation. Both tablets are combined in the same file.