I don't know about grinding glitter, it is colored metal after all. Grinding course glitter to make it finer will probably scrape off the color.
I've done symbol repairs with glitter before, and I use Martha Stewart glitter. The colors are very vivid and opaque. I have some colors in course and extra-fine glitter and I love them. I think either brand you found will work great. For pony glitter restoration, I would highly recommend getting the extra-fine glitter, it is just the right size.