I'm sure the Caswell dyes are much better than Rit. I have the Caswell black because the Rit black was being difficult with my constant voltage setup. The Rit dyes are just cheap ($4) to try a color out or do a single part. The Rit yellows and colors with yellow in them seem to go bad after use. I try to stick to the liquid dyes, but there are a few odd colors that I could only find in powder. I could get the Caswell kit and mix a color, but it's a bit expensive if you're just checking out a color.
You can mask with anything that survives the process. I've used rubber cement for splashes because it is viscous and makes nice runs and drips. Some of it comes off in my steamer, but I cleaned it up with mineral spirits or lacquer thinner (?).
I've masked the inside of regulators with furniture lacquer prior to stripping and it held up all the way through the process. That came off with lacquer thinner for sure.
That's a nice tank setup you have. I keep mine in the garage, so ventilation is no problem, but cooling is difficult. I had a circulating ice bath on the outside for a while, but I gave that up for ice bottles directly in the tank.