attaching D-rings to my M. Toulouse jumping saddle. It is the only saddle that fits Skipper perfectly right now, and we may be going on some long trail rides in the future where I would want to tie some things to the saddle to take with me.
The saddle pad I ride with is your typical square english schooling pad. If I were to tie things to the back of the saddle, would I need something thicker or longer to use under the saddle so that those things wouldn't rub? Or would it be okay?
The first issue is saddle fit, which sounds like you have taken care of with your current saddle (though do be aware that sometimes as distance increases, things do sometimes not work as they had at lessor distances!). As for what will happen with saddle packs.. a lot depends on the pack and how/where they sit.
I know some people (like me) who ride with their packs attached to the front of the saddle, meaning the packs themselves are touching the horse. Assuming the pack has a nice underside and is tied down in a secure way, rubs can be minimized. However, I could see a sensitive horse perhaps still getting rubs even with a soft underside.
Packs on the back of saddles are sometimes touching the horse directly and some people like to have their saddle pad between the pack and the horse. Again, I think it depends on the horse and the amount of movement of the pack. The lady camped next to us at a recent ride went out with an easyboot in her cantle bag which rolled over.. leaving the metal clip to rub against the back of her horse thru the pack.. He had quite the rub by the first hold.
But she said she had used the pack previously with no issue, but had never had the boot in it before.
My suggestion would be to secure the pack to reduce the flapping as much as you can, and then ride with it. Pay close attention to how its acting and how the horse's skin looks under it as you ride.