These questions are highly dependant on what project you work on or what you were asked to deliver. A classical recording might need almost no compression for pure dynamics, a pop recording might be heavily compressed, a modern art piece might be needed to be overcompressed. Actually this is also a thing not only valid for VSL but sample libs in general...
As far as "mastering" goes - don't let Dietz or Bruce R. read this [;)] Mastering is a final step to an already finished (ie. fully mixed and produced) product. For VSL I found the levels overall quite accurate and constant throughout, so you'd maybe need to make adjustments only when mixing with other libraries. I'd do adjustments whenever needed in the first place. There's a saying that you can't fix a turd at mastering stage. If you need to EQ an instrument, then apply that to the instruments tracks, you can't fix it in the mix later. Sometimes the volume slider is a better tool than a compressor. Dynamic range is as said dependant on (more or less free) artistic choice.
I highly recommend you to take a few hours to browse through the mixing forums here, as there are many tips here that I guess nobody can't sum up for you in a small guide to the golden mixing ear. As these are mostly developing skills over quite some time you will see you'll get better at jobs once you've done it for a while. I don't know of any example where someone was able to achieve good mixes by only reading some tutorials... Prepare yourself for a grea learning curve that will unlikely ever end - there's always new things to check out or other methods at getting things done you haven't tried before...
All the best,
PolarBear