Is there a shop at FCO or Roma Termini that sells SIM cards?
My itinerary is as follows:
Italy (7 nights)
Spain (2 nights)
Italy (1 night)
Switzerland (2 nights)
UK (3 nights)
I would be using the SIM card mostly for voice calls to/from these countries as well as the USA, and maybe some texting ...
Must it be Italy? Where are you? When will you start?
Take a look at togglemobile.com which offers coverage in the countries you mention.
If you are/could be UK-based and need to call or text the UK, take a look at Three's Feel at Home service, which includes most of those countries with Spain coming in April.
There are several threads about these in this forum if you Search. (I'm away with limited facilities at the moment.)