The 15 gr load for a 44 in the Pietta manual could be a misprint, that is more like the starting load for a 36. You will not blow up a cylinder with a chamber full of real black powder, there's not enough pressure there. The original Remington would hold 30-35 grains of powder, and didn't blow up, and compared to the steel in the repro you have, it was barely more than cold rolled steel.
Walkers were known to blow up with 60 gr of black, due to the poor grade of steel. Perhaps even the shortened to 50 gr Dragoon cylinders were not strong enough, but the 1860 Colt and New Model Army Remington steel was far superior to the Walker and early Dragoon's metal, and the repros made today are substantially better yet.
I would worry more about the large air space left in the chamber with a 15 gr charge, as the loading lever ram cannot push a ball deep enough to reach a powder charge that small. In this case, the ball is classified as a bore obstruction, and that could damage the cylinder.
Placing caps on the nipples with your fingers is safe enough, but if you need to push them on more firmly, as in the case of a CCI cap, which runs a bit smaller than the Remington cap, size for size, I would use a wooden or to be fancy, antler bone push stick, not metal and certainly never the hammer itself, and not your thumb.