I will just be filling in; this is a little bit too large for the roots to get around. You can take a pencil or just a stick or your finger and try to work this down because there is a nice root mass on this plant so it will stop some of these pieces from getting down to the bottom so I am going to work those down there. There is a variety of mixes you can use. You can put charcoal in here to help drainage or you can use Perlite that helps retain a little more moisture. Some people use straight fir bark as this is. You would be surprised when you get into orchids, potting and fertilizing. You can read all the books and listen to different ideas but you pretty much base it on your environment, your conditions, what you have to work with, you will pretty much come up with your own plan. Orchids are very adaptable generally speaking and you can try a lot of different things with them and they let you know when they are not happy and then you can change to something different.