a gymno will not like JI and grit? Is there clay in it?
Almost anything will grow in almost anything
There is clay in John Innes, a small proportion but a necessary one. A soil without clay is called a beach.
I use grit only as a top dressing. It is heavy, I prefer cat litter. Others use the even lighter perlite. One advantage of a lighter aggregate, other than just being lighter, is that it becomes easier to tell the difference between a wet pot and dry one.
Gymnos, and some of the other South Americans, appear to do better in a soil with more organic material and a more acidic pH. Don't panic about regular JI#2, but be cautious with hard water, don't add limestone, and maybe try the ericaceous to see if it works for you.
Terracotta pots? Does anything dislike terracotta pots? I think you can grow anything you want in unglazed clay. Or plastic. Or anything else that holds soil, preferably with holes but even that isn't essential. Clay is heavier, tends to take up more space due to the traditional shape, may be harder to find in a short wide shape, and is more expensive. Advantages might be a more pleasing appearance, but the ability to let through water is the big difference, so clay pots dry out faster than plastic ones. One thing you might not notice until several years down the line, roots adhere to clay pots and it can make repotting a bit of a nightmare.
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