I run Promatch 5000 mah with a 25C. The only problem with the batteries is that you have to remove the top deck and the battery tray to install them. The battery can be removed if need be, its just a pain. I charge the battery in the car and store the battery in the car.
I know that some people are running shorty packs and have good success with them.
On a postive side at least you are not putting a lipo battery into the mid motor JRXS; not that was fun.
Chris