You'll see info on the P.I.T. at the FCO airport in this post,
Terminal 3 Arrivals at Fiumicino Airport.
The buses are corwded during peak periods and on the more popular routes, like the 64 or the 40. (Or Anything headed to Roma Termini!). After a while, we just adapted to the crowds and bus riding was our primary mode of transport when we were not on our motorino.