Parakeet grit is perfectly fine. Many people will caution you about calcium, but to be honest, I've never had a problem with any of my batches I've fed it to. I think it has to do with the fact that the chicks don't eat much grit, just what they need. (unless they're a day old, then they try to eat their own toes, the bedding, the walls... silly things)
I would avoid anything from outside unless you are giving them medicated feed. Cocci infections come from contaminated soil, and pretty much any soil outside is contaminated.