So is the surcharge applicable for the return? (Ostiense to FCO) I'd bet it would be
You won
It's applicable to the return. If there's the airport in the itin, then the surcharge is applicable.
So, 8 + 8 euros
Do these trains (the FM1/FR1 departing from FCO) have a tendency to be packed like the Leonardo Express can be?
I take the Regional to/from FCO very often and based on what I see at FCO, the LE is more crowded. It looks like most of the (foreign) travellers take the LE disregarding costs and destinations, so the Regionale is more used at FCO by staff, Italian travellers or people that are more aware of the service.
anyway, the regional is a mass transit service, so when it's get closer to Rome center, especially from Fiera di Roma if there's some main exhibition or from Muratella where a lot of big offices lay, it can get crowded during rush-hours.
BTW, at FCO you can have easily your seat, so that's not a problem.
And regarding luggage I'll just have one rolling carry-on size bag. Would I be able to keep it with me by my feet, or would I have to stow it overhead or elsewhere?
Usually you keep by your feet. Based on the equipment, the coach will look like
this one
or - unfortunaltely, more often - like
this one
PS: it's only my impression, but I'd say that the lower floor is usually better air-conditioned, the upper floor has some design problem re the heat insulation; and it's reaaally hot in Rome now!