We also pad for rocky work, Our farrier explained that the pad takes away the horse`s "feel" for the ground and they allow the shoe to slip a bit more. The pads tend to compress as they are stepped on and the pad puts the shoe further from the hoof allowing it to shimmy on the foot. (Definitely not goos for the sidewalls.) Your shoes may not stay on as well with pads. What helps us are side clips. We don`t use the pre-made side clips as they tend to be too short. Our farrier pulls them himself so that they get over the pad & up onto the hoof wall to provide the needed stability.
As for pads on the back, our farrier tells us that as the foot lands, the horse needs that feeling to find the best purchase, and then it pushes off. In addition, the horse allows the foot to slide a bit to absorb some of the concussion. This need for feeling the ground and the increased slippage causes problems with losing footing and losing shoes if they are padded in back. The nails have a lot of shear stress and a longer fulcrum by going through the pad. (Think of the horses that are sore from borium on back shoes - not all as some learn to adapt quite well. Their feet slip and the concussion & strain is thrown onto their joints & muscles. They are intended to slip the back foot a little.) Also, the majority of the horse & rider`s weight is on the forehand where padding tends to help (even if riding in a center position.
The frog is stimulated and improves circulation to the foot by ground contact - the horse will loose some of that with pads. It is also hard to keep the foot clean under the pad. Oakum is the traditional packing, but it comes out with the miles that we do. Many farriers are using silicone to fill the pad, but it also sheds. One Old Dominion rider/corrective farrier (John Crandell) cuts an oval shaped, small hole out near the toe so that dirt & water can filter out from under the pad, and uses the opening to apply thrush medicine to prevent fungal growth in this nice, warm, wet environment under a pad. I haven`t tried this - I suspect problems if riding on gravel, but works great for large chunks of `rock`. DO NOT try hospital pads with a pop out centers. They are for sedentary horses too sore to walk! They will fall out every few 100 feet. Not a way to get many miles down!
We "discovered" Shock Tamer pads some years ago - they`ve been heaven sent! They are a composite pad, much like the sole of a running shoe, yet quite thin. They make full & rim pads. (Didn`t see the marked difference with the rim pad that we did with the full pad.) One horse had a ST on one foot and "brand X" of the same weight & thickness on the other front foot since the farrier had run out of ST. There was a NOTICEABLE difference in his comfort. I can`t speak highly enough of them!
I would pad the front only, and try rim pads to see if that was enough before going to full pads. The oval cut out may be an option to decrease thrush & sand buildup, but I haven`t tried it myself yet. John is VERY successful with it, though.
Schock tamers are the way to go. I have used the silicon packing that you can order from Overland Farrier Supply 800-838-5277. It is specially formulated to use with horse hooves. I fill the pad til it squirts out the back then i duct tape the hoof securely and make the horse stand on a level surface (mats or cement flor) for 2 hours. I leave the duct tape on til it wears off.
I did 5 days of the XP this way with NO SHEDDING at all. When we pulled the shoe and pad after, the silicon was perfectly formed to the frog and there was NO sand, dirt or thrush in the hoof (7 week old pad/shoes).
Another alternative to okum and silicon for packing when you use pads is a product made by Hawthorne Products(1-800-548-5658) called SOLE PACK. It is excellent since it medicated to relieve sore feet and combact fungal infections. It also seems to stay in the pad and the consistancy allows more to be inserted if necessary.