Succulent help! Over-watered?

I recently bought a beautiful arrangement of succulents from a woman who sells them professionally (see attached photo). They really are gorgeous, and although I'm definitely still a beginner I've become very interested in succulents and was hoping to try propagating them as soon as possible.
I live in a college dorm, so my space and access to light is limited. My window gets indirect sunlight. I've noticed a few of my plants getting a bit leggy, and I assumed that this was the reason.
However, I have recently noticed that a few of my plants are turning yellow on the very lowest leaves, and some appear to be slightly deflated and mushy. None browned, yet.
I have done a good bit of reading on the internet and my guess is that I overwatered them. I was warned not to do this, and thought I was being careful, but I totally blanked on the fact that this lovely arrangement came in a ceramic container, with no drainage, so I am guessing I did indeed over-water them.
Obviously I will not water them again until I can be sure the soil really is dry, but I don't know if that will be enough to save them! Is there anything I can do?
Please advise me how I can save my poor over-watered plants, I don't want to lose them! :(

The problem with a mix like that is that not all succulents need to go quite so dry between watering, but some are so susceptible to the root rots that they must go very dry between watering.

The other problem is that none of the plants have enough room to spread out and grow.

I would
a) not buy anything like that, unless you want all the plants and it is a good buy.
b) divide it up and plant each one separately. Depending on what size they are, anything from 2" pot on up might be about right. I would have several sizes of pots on hand, just in case you find a bigger root system on one or two of them.

To make a 'garden' like that I would find a shallow container and set each pot in there separately, then sprinkle some pebbles around for decoration.
Then, if one of the plants did not like the setting it could be pulled out a lot easier and something else put in its place.

Thanks for your advice Diana!

I really like how they look next to each other, and I think your idea of potting them separately but putting the pots next to each other is great.

Is there any way I can find out how much watering different varieties of succulent need? I've tried to identify them but had a very hard time. I'd still worry about mistreating them if they were separate, since I'm not sure about their individual needs.

The loss of the oldest leaf on any plant isn't cause for alarm, without any other symptoms of distress.

There is a whole genré of gardening that's all about combining succulents. I don't have any succulents in lone pots, and find they do best if they have companions to use moisture as quickly as possible. I would encourage you to take a look at the sticky at the top of this forum. Some of it wouldn't apply to succulents, they don't generally grow enough roots to trim, but the parts about soil/moisture applies to any plants. If your soil isn't causing roots to rot, there's no need to treat each plant differently, just water when dry.

The one little prickly plant in the middle looks like a real cactus and might do better if scooped out and separated so it can spend most of its' time dry. Combining cacti and succulents is going to another level.

If your container doesn't have a hole in the bottom, the risk of adding too much water just one time is extremely high, I don't try that for any plants. In addition to a soil choice, unglazed clay (when choosing from traditional 'flower pots,') can offer the most possible air to roots, quickest possible drying. I've been buying orchid pots, that are unglazed clay, not as tall as the traditional pot, and has slits on the side for even more air/quicker drying.

The pic to the left, that's not even a pot, it's a wire mesh basket.