50/50 soil/grit should be good for the amendment (to be mixed 50/50 with native soil). A lot of succulents have pretty shallow roots (and do well in smallish pots) like your Aeoniums, but given a chance to expand in the ground they can root deeper. Sure, 10" is good. Maybe 12" is better.
Regarding pumice vs. perlite: the main difference is perlite is really light and will blow around, and probably be a bit messier because of that. I don't know if either is particularly good for the environment. Pumice comes out of a mine. You can also use lava rock, or various other kinds of fine gravel, depending on what's available to you. Avoid limestone, avoid shells, but pretty much anything else goes. Even construction sand can work for this purpose if you screen it first to remove the fines.
