You're doing okay- the sponge thing in the mix is normal. Sometimes they use that.
But as you notice: you never know whats hidden in the pot. And if you dont know what the media is, its really hard to water correctly.
I would repot them all, start with the ones that need it the most, like you did. After a repot you know all the roots are healthy, and you know what media they are in.
When I repot, and I find a plant with not very much roots, I like to keep it in a vase while recovering. Mist it every day. You can put it in a pot- with new media. (I have a repotting thread for my phalaenopsis- if you want to search and find it, lots of pictures). Yours seem to have enough roots to just repot after removing dead roots.
New pot. New media. Water, and make sure it drains well. Hopefully they will bounce back in no time
Edit: can use the old pots, just clean them. Just meant giving them a new start.
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