The problem is most likely starting from the soil - from the little I can see it looks pretty dense. Can you get some perlite to mix in with your soil? That would help a lot. Mix heavily - say 60 - 70% or more perlite and the rest potting soil.

Does the pretty pot have a drainage hole? That will be key as well.

Are the plants getting any sun or are they indoors only? If not, they need several hours of sun a day (acclimate slowly or they'll burn)

Also, did the roots look good when you potted them up?

I think if you can adjust the things mentioned above your plants should do much better. I hope this helps,

There is some perlite mixed in, but not a lot, not 60-70% for sure. (it was a premix for succulents). It does have a drainage hole, it normally sits by my office window, where I've been giving it indirect sunlight with the blinds about half open since planting. They get sun from mid morning to late afternoon where they are.

Roots looked fine when potted. I will try changing the soil and see if that helps. I really don't want to kill my first try at succulents!

If I could give you a tip: repot, into better mix as elucas suggested. Rinse or sift perlite before using to get rid of smallest particles & dust. Double check roots at the same time - get all old mix off them.

Since they are your first succulents (as you said), use different container. This one is sealed, looks very nice but may not be easiest to grow succulents in. So maybe you can start making it easier on yourself and the plants, use a terracotta or even plastic: these do not breathe either, but have many drainage holes. As you gain more experience, you'll be able to use different containers. Just an idea...you don't have to do it.

One more question, for topping material, should I be using something smaller than these flat marbles? They're the average size that you get at any craft store, you can see they leave gaps.

You can leave those, they are OK for topdressing.