I think the 5.50" (or anything shorter than that 8" barrel) .44 Remington suggestion is pretty good. Don't overlook the .36 Remington either, they are more powerful than you think, and nice to carry. I've switched from a 1860 Colt to a Remington Navy lately, and I think that with 777 the .36 is as powerful as the 1860 with Black, and I don't really think the Colt design is "up to" full charges with 777. Again, the .36, actually a .375, is not a weakling, and are very comfortable and unobtrusive to carry.
It's always a good idea to carry a sidearm. Even if it's not strictly legal to use it for the coup-de-grass, it sure beats blowing another rifle sized hole through the animal if it's still alive. I also think that since you never know what kind of two legged varmint(s) you may run into out in the out-back, it's a good idea to have some extra fire-power on you.
I would think that wonder wads alone, and skip the grease, will prevent any chain-fire, if the pistol is not defective. I too have shot cap-n-ball for many years without ever a chain fire.
When packing a rifle, I'd rather pack a pistol that's comfortable to carry and does not get in the way, so IMHO that kind of rules out the ROA and the Walkers and Dragoons. With just a 1.50" shorter barrel, my Navy Remington is much more comfortable than my 1860 Colt.