Howdy
This question comes up a lot. I have two 1858 Remington, one is a EuroArms import that I bought brand, spanky new around 1975. The other is a much more recent Uberti Stainless. I have R&D 45 Colt conversion cylinders for each of them, and in fact I never shoot them as C&B anymore. Yes, the Stainless Uberti did come with a blued conversion cylinder.
Personally, I only fire Black Powder through these pistols, but yes, you can shoot Smokeless Cowboy loads through them. Here is a scan of the little pamphlet that R&D supplied with one of the cylinders. Notice it says NOTHING about pressure, because there is no official SAMMI standard for Cowboy ammo. But if you duplicate cowboy loads that can be found in just about any modern reloading manual, you will be fine.
P.S. Yes, there is often discussion on just how high you can go with these cylinders. I have no idea what they were proofed to. Common sense says to keep the loads to Cowboy loads, and you will be fine.
P.P.S. Actually, I usually shoot 45 Schofields in my Remmies. The grip shape of this model is a bit different than that of a SAA, and although I can shoot full power Black Powder loads with 250 grain bullets in my Colts all day long, they are a bit uncomfortable in the Remmies. So I usually shoot them with Schofield cases filled with about 28 grains of FFg and a 200 grain Big Lube bullet. Still plenty of boom and smoke, not quite so punishing. 45 Schofield is a good round to think about if you want to shoot Smokeless in Conversion cylinders. Originally, the R&D cylinders would not accept the larger diameter Schofield rims, but today they do. And 45 Cowboy Special is also a good load to consider in conversion cylinders.