Should I vary the first layer height
Default settings for first layer height in Slic3r Prusa Edition print profiles regardless layer height is 0.2 mm.

If you need to improve bed adhesion then try tips from this video 3D Prints not sticking anymore? Watch this! 3DP101 by Maker's Muse. It's about using glue stick and spreading it using paper towel and isopropyl alcohol. 

There are other possibilities how to improve bed adhesion, e.g. Ultem sheet or other printing surface like BuildTak.