You can tell when they've outgrown hides because they look like tortoises- you can see the snake sticking out underneath the hide, which is lifted up on top of them!
If you get hides that are too large for a while, you can stuff some paper wads in them to make them cozier while they grow into them. Mostly BPs like to feel sorta snug in their hides.
I've never used it, but I agree from what I've heard/read, that Natural Chemistry mite spray should be harmless & fine to use "in case". I just wouldn't use the "PAM" (provent-a-mite) stuff, or anything along those lines.