Howdy
The reason you do not want to cut down 45 Colt brass to 45 Schofield brass is that the wall of the case gets slightly thicker further away from the case mouth. It probably will not make much difference with cutting 45 Colt down to 45 Schofield length, but it might make crimping just a tad fussier.
I buy all my brass from Starline. I checked a few days ago and 45 Schofield was back ordered, but it looks like it is back in stock now.
Not much gas vents out the nipples, that would not make a significant difference in recoil between a Cap & Ball load and a cartridge.
However, the weight of the projectile will make a big difference, as will the crimp. There is not much mechanical retention holding a round ball in the chamber of a C&B revolver, just the surface friction of the very small cross section where the lead was shaved away seating the ball. By contrast, the crimp in a cartridge mechanically locks the bullet in place. Everything else being equal, pressure will build higher with a cartridge to push the bullet out of the case than with a round ball seated in the chamber of a C&B revolver. The higher pressure will result in higher felt recoil.
I do not know what brand of conversion cylinder you are using, but the current cylinders that Taylors sells for 45 Colt conversions in 1858 Remington Cap & Ball revolvers are cut large enough to accommodate the larger diameter rim of 45 Schofield brass.
Personally, I don't think it is a great idea to be firing cartridges from a brass framed revolver, the increased recoil will pound the cylinder back into the frame more than a mild C&B load. You may experience some stretching of the frame, or the ratchet teeth of the cylinder may dig impressions into the recoil shield.
For what it's worth, my 45 Schofield cartridges are loaded with 1.9CC of Schuetzen FFg (about 28.5 grains) under a 200 grain J/P 45-200 Big Lube bullet.
In this photo, the cartridge 2nd to the left is one of my 45 Schofield Black Powder loads.