Welcome to the forum. I would move the cattleyas to a spot where they can get much more light. Cattleyas need fairly bright light if you plan for them to rebloom. Oncidiums like 'dancing lady' get little brown sun spots all over their leaves. This is just how they grow. If they get enough light to bloom, they will have a few spots.
To answer you potting question I would have to know your watering schedule. For me, I would leave everything in bark, but that is how I grow. There are others here who do grow in spag and are very successful with it, so I will leave it to them to respond on that point.
Good luck with your new plants & welcome!