Hiyah Ivan. Im making a mirror at the moment and have wound up using 19mm thick plate for a 250mm (10") mirror. Grinds out a LOT faster than pyrex. People tell me thats on the thin side, but if i pay attention to keeping everything turned around often during the grinding i should get a decent mirror out of it and avoid astigmatism. The telescope i bought 12 months ago has an 8" mirror made out of 19mm plate and that is a really usefull little beast. A guy at a glass supplier i spoke to a few months ago said he sells a bit of 15mm to people doing 6" mirrors.
Hey, im new at this too, but i think that if 15mm was all i could get i have ago at an 8" with it. Dont know much about the grinding powder issue. The grits have to be REALLY consistent size / clean!! One piece of #80 sized contamination in what you think is #400 can ruin can ruin a lot of work by scratching it. Our group got their grits from a lapidary (rock/gemstone polishing) supplier.
Anyhow, have fun with it however you go.
Seeyahs!