1. Termini is easy to navigate. The platforms are arranged in sequential order along a line, so should be easy enough to find. The usual trap for unwary players are the two platforms 1E and 2E, which are at the so-called ‘piazzale est’ that is half a mile from the main concourse, and 2E is especially tricky, because it is easy to assume that it is at the end of Platform 2, but if you go all the way to the end, you’ll get confused at not finding it, and eventually you’ll just have to double back to the concourse before making the 1/2 mile trek out to the ‘piazzale est’. I do speak Italian, so I have not paid attention to clarity of signage for non-Italian speakers, but I can’t imagine that there should be too many problems, since it is clear where the platforms are (apart from the aforementioned 1E and 2E), and then you’re just looking for numbers.

2. Fifteen minutes is usually ample time to transfer between trains.

3. This would very much depend on the train company in question, and the fare level you buy, i.e., whether you get the base fare or a discounted fare, and whether it is a reserved train (i.e., train with reserved seats) or not. Again, details are important here. If you’re connecting between two Trenitalia trains, for instance, generally, if you’re connecting from one reserved train to another, a 15-minute connection time is required for the connection to be protected. If you’re connecting from an unreserved train to a reserved train, in principle, 60 minutes is required (depending on fare level). Taking the Leonardo Express from FCO, if you miss your booked train, you can simply take a later one with the same ticket, but if that causes you to miss a connection, you will of course not be protected. The Leonardo Express is moreover unreserved, so, in principle, a 60-minute connection time is required if connecting to, say, a FrecciaRossa. This can be waived by station staff at their discretion, but it is important to see them before boarding another train.

It really would help if you provided details of your itinerary, e.g., time & date, which train company you’re travelling with and which online platform you’re using to book. For instance, you ask about the business of the station, but that really would depend on the time and the day. I don’t understand why people insist on being vague when they’re seeking help. For instance, in another thread, someone posted that they were flying from ‘a US airport to FCO’. Why the vagueness (not that there are so many possibilities)? Details that you think irrelevant often aren’t. Help others to help you! All right ... rant over.