Biggest difference....RC has organics and anti-caking agents in it. It needs to be stirred every so often, or it will go anaerobic. I use RC and make a big batch to store, in case I need to make an emergency water change. I keep a small powerhead in the container that kicks off several times a day.
If you are just mixing to use and not to store, then the aforementioned is not an issue. Combining the two is not an issue. Personally.....I would pick a brand that works for you, and stick with it. Once my RC is used up......I am going to switch over to the regular Instant Ocean....it mixes up better, and doesn't need to be stirred periodically. I dose Kalk in my ATO, so the added Alkalinity and Calcium is not needed for me with the Kalk supplementation.