The Swiss Pass covers travel in Switzernald. It does not cover travel outside of Switzerland. Forget however what was said about the Eurail Pass, as that would not be a good option for you in this case either.
For your first trip, Paris to Grindelwald, just buy a ticket Paris - Basel. Than in Basel you just continue to Grindelwald, showing your Swiss Pass to the conductor when requested.
Regarding the trip to Florence: The way to go about this is to just book Geneva - Florence with SBB, and indicate that you have a “GA” (General Abonnement). That way you will not be charged for the Swiss part of the trip.
Which places are your visiting in Switzelrand? Geneva is not a good place to end your trip if you are continuing to Italy, because of the works on the Simplon route. If you can rearrange your visit so that you end up somewhere near the Gotthard route you will have an easier onward trip.