KK, The one on the left is a Mammillaria, but I can't help with the species. At its right is probably Parodia (Notocactus) leninghausii. In back is probably Orerocereus trollii. You can probably keep your new seedlings in their humidity dome for as long as you want to. Most cactus seedlings are happy with 100% humidity and wet roots for at least year, longer for some. Once they start to touch each other, it's best to repot them, just so they grow faster. HTH, -R

Depends on who you ask, but it's commonly called Acanthocereus tetragonus cv. "Fairy Castles," it requires typical cactus culture i.e. mostly inorganic soil, full sun, water when soil is dry, do not over-water or leave in cold wet conditions.

It could be insect damage, who knows. Put the plants into "winter" mode and give no water for several months. There is a good chance it will heal itself. You have a lot of offsets so the plant is basically healthy.

Oh man this does not look good at all. Your cactus is a Neobuxbaumia euphorbioides and I have found these guys to be super sensitive to wet and cold. Especially when young like yours. As they get older they become a little tougher. I had three young specimens that died last summer, which is the growing season, from too much moisture and not enough airflow. So too much moisture during the cooler months is not good. I’ve got an older one that can take a little more abuse. This guy I doubt will make it🙁. I can tell the copper/dark area is inside the cactus and not just on the surface. Unfortunately that does not heal on its own and will just spread to the healthy bits, as I’m sure you’ve noticed. You can cut the unhealthy rotten parts off, but in your case the bad parts are vertical and all over, and not just in one area. Plus i wouldn’t be surprised if the rot has made it to the vascular ring. Not my pic, from Google images. The vascular ring is like the cactus’ circulatory system it carries nutrients and water from the roots to the plant. So when rot reaches it, it also aides in the spread of rot🙁. I would def take them all out of that damp mix. Check the roots because they may be good. You might be able to save a few inches above the roots if there is no rot. How long have they been like this? Whenever you see rot on succulents it’s best to act ASAP because it doesn’t go away. Spreads very quickly. Every once in a blue moon the cactus will heal itself, but sadly yours doesn’t look to be healing. When and if you unpot and you want help I or someone else can help. Sorry to be the bearer of bad news, but it’s all a learning lesson.