A Desantis Nemesis meant for a J frame will work for a DS in a pinch, but not all that well and won't likely be entirely satisfactory. You might as well just cram it into one meant for a Baby Glock (which also will work for revolvers, sort of, since the Desantis are somewhat generic).
I wasn't satisfied with the results, and simply went with a hobo rig - cardboard, Gorilla tape, and a tensioning #64 rubber band. CheezIt boxes work well. This was the same method that I used for holstering my Model 12-3 when I had it and what I use with my Walther PP. It's the easiest way to get a pocket holster for less popular frame size guns.
One of the alloy frame Colts might work better in a Nemesis, I don't know.
K frame speedloaders do indeed work for a D frame Colt, at least HKS ones do, but not perfectly. HKS makes ones specifically for the Colt D frames, and these are the ones that you want if at all possible. The use of K frame speedloaders was most common in the days when people carried a 4" K frame as their primary and carried a DS as their BUG. The K frame speedloaders could be used as an emergency reload in a pinch.