You can use .454 inch balls in any revolver that uses .451 inch balls. You'll get a slightly larger ring of lead when seating the ball, but the difference wont' strain your loading lever.
I'd go with .454 inch balls. I've found the larger size more accurate in all my .44 cap and ball revolvers. I believe it's due to the larger ball creating a slightly wider bearing band for the rifling to grip.
For best accuracy, use a greased felt wad between the ball and powder. Doing so, you don't have to use grease over the ball; the lubricant in the felt wad provides all you need.
What lubricant? Use Crisco, lard, SPG, Lyman Black Gold or any natural grease. Don't use petroleum greases. In my experience, black powder when coupled with petroleum greases and oils produces a hard, tarry fouling.
Maximum load for your 1860 will be about 35 grs. of Goex FFFG with a .454 inch ball, without a greased wad. The greased wad will take up about 5 grains of space, so you'll be able to get it to fit if you drop down to 30 grains.
The Colt design is a much better natural pointer than the Remington. The Remington is stronger, but it's moot when black powder pressures are involved.
All Colts I've encountered shoot high at 25 yards, as much as 12 inches above the point of aim. They were designed to do so, as they were often used to augment the rifle out to 100 yards or so.
Taking your time, you can easily hit man-sized targets at 100 yards with these pistols. A good pistolero can hit the same targets out to 200, or give them a good scare!
Don't think for one minute that these pistols are toys. They're just as deadly as a modern arm; the ghosts of millions killed by cap and ball revolvers throughout the world nod their head in agreement.
Use FFFG black powder if you can get it; I've found it more accurate. Pyrodex works, but is more corrosive than black powder.
Which leads me to a final point: Clean your revolver immediately after firing it. Black powder, and its substitutes, are corrosive. They will promote rust within a day in damp climates, and in days elsewhere.
Clean it with hot, soapy water. Remove the cylinder, and then remove the nipples from the cylinder. You'll need a nipple wrench to do this. Remove the wooden grips, set them aside.
Now, strip it down to the last part and scrub everything thoroughly with brushes of varying sizes. Q-tips and pipe cleaners work well to get into tight places.
Rinse everything well, shake off the excess water, and place all metal parts in a shallow pan. Put the pan in the oven, set to its lowest temperature, and leave the door open a little to allow moisture to escape.
Thirty minutes in the oven will drive out all moisture. Re-oil with olive oil and reassemble. Put a little olive oil on the threads of each nipple, for easier removal next time.
Because this is a new, unfired gun I'm suggesting you give it a thorough cleaning. You'll likelyl remove all kinds of factory crud in the bargain. Depending on the humidity of where you live, you won't have to detail strip it each time you take it out: just clean the cylinder, bore, rammer, frame interior and sides of hammer with a damp cloth.
Then oil again with a little olive oil.
Eventually, it will begin to get harder to cock and drag, and it's time for the hot water bath and oven treatment again.
The Colt is a simple mechanism. You'll find all kinds of instructions on the internet for breaking it down. In no time, it will become second nature.