Seedlings don't have the water storing capabilities of mature cattleyas with large pseudobulbs. Mature cattleyas like fast drying medium but smaller cattleyas should be grown in a medium that holds a little more moisture. Sphagnum is fine if you're confident about growing in it. I have problems with sphagnum so I like a mix of fine fir bark or fine coconut husk chips with a little sponge rock and charcoal. Don't plant seedlings in the same mix that you use for large plants. They need a finer mix. Growing cattleyas from seedlings can be an exercise in frustration because they often take a couple years to get to blooming size. Good luck.