That's too tight a schedule, or would be for me anyway. It's possible if the passport control line is quick, but it completely depends on staffing that day, as well as how many flights are coming in around the same time. I'd usually say the passport country matters too, but at FCO, the EU line isn't always faster than the other passports line. But your bags will likely be out by the time you clear passport control. If you really want to aim for a 9am tour, you'd be better off having them pick you up at arrivals and going from there and just taking your luggage with you.

Otherwise, the safer plan is to assume you'll be arriving into Termini right around 9am. Add another 15-30 mins to orient yourself and find a place to store your bags. I'm not sure if there's still one within the station itself, but there are 3 baggage storage places within a couple of blocks west of the station. If you go that route, having your guide meet you at Santa Maria Maggiore will get you out of the immediate vicinity of the station. If nothing else, fewer people will try and sell you .... while you wait.

I just got back from Rome and was staying in Monti for a couple of weeks, so feel free to ask more questions about the area if you have them.