Holds both Lipos and mini AEG batteries.
Unless you stick a lipo in your RIS I do not think it will be able to fit. There are however, hand guards designed to fit small to large type batteries with rails attachments, like this Guarder SD hand guard.
As for hooking up your batteries in parallel, you now have to lug around 2 batteries in the front end of your weapon. I don't see a point of connecting 2 batteries in parallel if you can just swap a fresh battery in under 10 seconds.