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Most Mammillarias tend to prefer a gritty compost. I don't know what proprietary composts you have in NY, but I use 1/3 of a general plant compost (in UK we call it John Innes), 1/3 horticultural grit, and 1/3 volcanic pumice grit. The key, I believe is an open, free draining mix.
PM me if you have any further problems.