Wolfie, what kind are you buying....they have several different types. I purchase the large 2 cu ft. bags and mine is so dry....I will check out what it says on my bags. It even has some stuff in it that looks like wood bark. I have better luck with that than with the professional stuff I was buying to save money last year.
Pegie, you do not need to put them under a grow light. Just put them in a good soil and water them but do not over water....I keep mine in the greenhouse under the shade cloth and they get watered with the rest of the plants but I never water too much at a time. This heat makes it difficult to say how much you should water. I touch the top of the medium and if it is dry and looks dry, I water. I use a bit of root powder and just let them go. I really make them wet when I first put them in medium. After the first day or so when they begin to look dry I begin their watering with the rest of the plants. I have hundreds growing now and some are getting too much light but I can not put another shade cloth on. They will get pale leaves but that will go away in the winter months.
When you take off a piece of Christmas/Holiday cactus, remember, do not use a knife etc. twist it until it comes off. Then, most people let them callus for a few days before planting.
On the other hand, some people I know, including me depending on which species I am cutting stick them in water and they will even root in water. I do not recommend you do this. I do it when I have so many I am not sure when I can get them in medium. It works better for me...it may not be the best way but I always say, if it works for me if is my way......that does not make it right.
My Cubits has a forum for each species I propagate and I have started to put bits of info I have picked up over the years under the specific species if anyone wants to take a look I would love to share it with you.
As I said before, the above info works for me, that does not mean it will work for you but it is worth giving it a try. Have fun.