I see a few tags with "Pisgah" clones and a few other tags that look similar to what Looking Glass Orchids (formerly Owen's) uses. Is that where you got them? I had one of the Pisgah Fanaticum clones, but it was in rough shape when I got it (he wouldn't even accept money for it) and it ultimately passed on to orchid heaven. Looking Glass is a fun place to visit. It's relatively unknown, but he's got bench after bench after bench of wonderful Paphs and Phrags.
I've visited and gotten some stuff from Looking Glass and I think some of the plants were/are in promix. Many/most slippers will do just fine in promix. It's peat or coconout based, and a little different from traditional houseplant potting mix. It's more popular with commercial growers, because it's relatively inexpensive, readily available, and helps reduce watering needs (i.e. labor costs). I've seen commercial growers successfully using promix or similar soilless mixes for Phals, various intergeneric Oncidiums, even things like Dendrobiums.
That aside, I've found it hard to water correctly in promix, so I'd also repot ASAP whenever I get plants potted in it. I've used a rockwool based mix for my paphs and phrags with lots of success for many years. I've only recently started moving stuff into orchiata, though I still grow lots of things in rockwool. My reasons for switching were that it seemed harder to keep things properly fertilized in a completely inorganic mix, and I ended up getting what I can only assume was a bad batch of rockwool that nothing seemed to want to grow in. I always wash and soak before use, and make sure to check both TDS and pH with rockwool. While those parameters tested normal before using the stuff, I think there was something I didn't test for that was off about it.