There was recently a discussion here concerning file setup: Best Practices for Obliques in the Same Glyphs File?
Regarding spacing and kerning, if you plan to only slant (and optically correct) your glyphs, I would say you can copy/paste your kerning for the most part. Of course, ideally, you will have adjusted kerning as well, but this depends on your design and how much work you wish to invest.
If you want to transfer your spacing to your slanted master(s), I wrote a script for specifically this use case. You can find it in the eweracs scripts (available in the plugin manager), Drawing > Copy Metrics to Slanted Masters.