Hi there.

As a rule I'd rather avoid doing a self-connection as if, for whatever reason, you arrive too late for your onward flight then you're out of luck and the airline won't help you. You'll be classed as a no-show and you'll need to fork out a (usually hefty) fee for another ticket.

Anyway, with regards to your questions. Bear in mind all of the below info is from the airport website itself: https://www.adr.it/web/aeroporti-di-roma-en/

Terminals
Your UA flight (I suppose either from EWR, or ORD, or perhaps IAD) will arrive at Terminal 3. Ryanair and ITA's services to BRI will depart from Terminal 1. Most US services arrive and depart from gates E31-E44 in T3, which is the remote satellite connected to the main terminal by an automatic train.

Last time I checked, but admittedly it's been a while, there is an airside walkway between Terminal 3 and Terminal 1.



Security & customs
Yes to both. You will need to clear customs first, then security. You can stay airside, as far as I can remember, if you're not collecting luggage and you're already checked in for your next flight.

MCT
Theoretically, airlines sell 1h / 1h30 connection journeys through FCO, however since you're on a self-connection I wouldn't go with anything less than three hours. Especially if yours is a return trip and you're planning on doing the same journey in reverse. If you miss your transatlantic flight it'll be a steep price.