Its best to mix it. Ironically having it sit at the bottom creates a consistently wet environment that promotes bacteria and rot. Mixing it with soil creates pockets of air which the roots use to breathe well and aerate.
Yes, the saucer is separate. There is a drainage hole in bottom with a plug that you can use if you prefer. It comes with a cut to fit mesh for bottom so that you don’t lose dirt out hole when you water the plant. It also has what I call “gravel” for bottom as you fill with dirt and your plant. It is very nice pot
The foam goes on the bottom of the stand legs! :) I used maybe one or two to fix the wobble mine had. Its nice in case you just dont want the stand to scratch/rub on surfaces
I would not hesitate to buy the 10 inch pot! My Fiiddle Fig was only 4/5 inches when I transferred him to the 10 inch pot. He’s now 17 inches tall.