A thin layer of booster is when you make the brush as dry as possible, dip it just a bit at a time, and then spread the booster so that there's barely enough to wet the entire surface of the sponge.
A thick layer of booster is the thickest you can spread it
evenly across the surface of the sponge. If you can't spread the booster evenly, that's too much booster and you should brush the excess off of the sponge.
Same goes for glue. If you can't spread it evenly, it's too thick.
Yes -- DHS No15 and Nittaku Finezip will peel off in one clean piece. These are the glues to use if you want to reglue rubbers. Here is a video of a clean peel (I think it is DHS glue):
Here is a video of a very unclean peel:
Most glues besides the two I mentioned will stick to the sponge more -- you can't peel them off very effectively and it might rip up your sponge if you try to peel it. It's fine to use them as long as you only want to glue your rubbers once.
You should peel the glue off any time you reglue your rubber, even if you don't reboost it. Peeling the rubber off your racket will usually create some bubbles in the glue layer, which can cause inconsistent playing if you try to use that glue layer again.