You will be limited by both chain length and the length of your track ends as to how many teeth difference you can run.
Consider what your requirements are first and that your hub is most likely not set-up track style w/ a lockring on both sides for FG so the "flip" side will be FW.
If it is track style on both, good to go FG/FG. Cogs are inexpensive as such things go, get a 19T and a 20T and experiment on the types of terrain you will encounter.

-Bandera