I am given to understand that the powders of 'days gone by' were substantially less potent than what is available today, and that the cylinders were longer to accomodate the greater volume needed.
This may be a complete pile of do- do, but it has always sounded plausible, considering the modern facilities and modern chemistry and modern quality controls that were unavailable to 19th century powder makers.
At any rate, i use fillers and i'm not ashamed to admit it. If you simply fill the cylinder up with powder, i suspect that you're in for an unnecessary surprise. I know of a fellow who did that with a Walker and was surprised that the resulting recoil caused a chainfire. He probably had at least 45 or 50 grains in each cylinder. Made an impressive cloud of smoke, though. Luckily, nobody was hurt and the revolver was undamaged (at least, as near as i could tell).
As regards spare cylinders, i have gone through the mental list of photos of Civil War types, and i cannot recall a single image which would indicate that the soldier was carrying a spare cylinder, although many of them show multiple revolvers. I would guess that it was just a lot easier to shove the empty revolver back into your belt and pull out another that it would have been to try to change cylinder while everybody was shooting at you.
Free opinions, and no doubt worth exactly what you just paid for them.