It's safe for most reptiles, but it doesn't do well in a terrarium environment. What type of reptile?

The second one is an Echeveria runyonii also known as 'Topsy Turvy.' I would not pot them together as they are all very different plants. It will be easier to monitor and nurture them if they're in individual pots. They each need a well-draining mix in a well-draining pot. The cactus and succulent mix you have will work well if you cut it with Perlite. A good place to begin is 50% Perlite to 50% soil mix. You can find Perlite at any nursery or Big Box store like Home Depot. It's relatively inexpensive. You can try beheading the stretched one. Let the cutting callous for a couple of days, then stick in the potting mix. Do not water it until you see growth. Keep the stem along with the pup. I'm not sure about this, so perhaps someone with more experience can chime in. When you do repot, add potting mix up to the top rim of the pot. That way your plants can get maximum air circulation. Good luck!

Fine sand does that, at least in my experience. I had been given plant potted in mostly sand, and it was as hard as concrete. Roots can't get enough moisture and can't breathe in that kind of stuff, and eventually either rot or dry up. It doesn't happen overnight, so it may be hard to notice. I never use sand, most ppl don't. Coarse sand is OK, but probably nothing that will pass thru the kitchen strainer - IMO, that would be finest I would ever use if I had to. Hard & compact soil isn't recommended either, best substrate for succulents is one that drains fast. Many ppl add lots of perlite(at least half of total volume) to their C&S soil; others (including me) use more inorganic mix. Grit, perlite, pumice, scoria, turface and similar are all great to be used to make well draining mixes. Very experienced growers use sand, but I really can't tell you what kind. More often than not, any black marks (dots, lines or circles) appear if there is rot. There is very good chance the stem got damaged and water was collecting there - obviously, I am just guessing based on what you posted. I would do exactly as you did, cutting until no dark/black is visible. Cuttings without roots should not be watered for a while as they have no roots to take in moisture. There is plenty stored in stems and leaves until roots start growing. Your plants are looking good, so your watering regime was probably ok for whatever they were potted in. In new substrate that you don't have experience with, you could check how moist or dry it is using a bamboo skewer or chopstick - insert deep into pot. If dry after taking out - time to water. If it feels wet/cold and/or has small particles sticking to it, there is still plenty of moisture. May I ask what is your general location? Looks like you have great sunshine available for your plants...

Have no experience of this brand as not one I’m familiar with here in the UK, but guess it’s the same as all branded c/s mixes....mainly peat based and not ideal on its own....but they never tell you that fact, you only find out when asking for help for rotting or dying plants! Needs amending with at least 50%seived perlite as the easiest to do. Or a 33% mix of c/s mix, seived perlite and aquarium sized gravel or similar for even better drainage. More experienced growers don't even use the c/s element but grow in pure inorganic mixes...as I said, you need the ‘know how’ before getting to that stage ;-) gill