Fifty-five minutes is very tight for a transfer at FCO. As mentioned, you need to find out if your carriers have an interline agreement for baggage transfer. And even if they do, the tight connection might mean your bag won't make it. I'd go with a carry-on if possible.

I'd also try to find out if your inbound flight normally parks at a remote stand, requiring a bus ride to the terminal. That process is slow and easily could make you miss your connection.

If your inbound does go to a terminal, try to get a seat near the door for quick exit and a quicker trip through security.