The first sector probably will not be a problem, however the flights to FCO are generally busy and ground staff will/should focus on handbaggage and so may the Crew. If
you are in excess of one cabin bag and a smaller bag (to placed underneath the seat in front of you, unless you are in row 1 or the emergency exit row) then be prepared to have the excess item checked into the hold for you.
Yes you may get away with it, taking up space other Customers may need too, but you shouldn't in a perfect world