For the timetable it is better to use classical DB website -
http://reiseauskunft.bahn.de/bin/query.exe/en?newrequest=yes&protocol=http:&, it perfectly shows Italian timetable and you can see needed train changes there as well.
With specific fares within Italy it is not that simple, actually Roma-Pisa can be easily seen on TrenItalia website (in both Italian and English versions) since there are direct regional trains, with Pisa-Venice it is a little bit harder - we can add up sum of separate tickets Pisa-Firenze, Firenze-Bologna, Bologna-Venice (all those can be seen on TrenItalia website), but then we need to count "break" of tariff, thus actual one through ticket will be cheaper than 3 separate ones. Plus in your case it was much easier since distance Roma-Pisa by train almost equals Pisa-Venice by train.
Ideally there are special tariff book for regional trains, but we aren't able to find it at the moment.
There is no need to book in advance - no seat reservations required for any regional trains and fare stays always the same (it doesn't depend on time of purchase or any other circumstances).
We as well would like to remind you that ticket for regional trains in Italy is valid for maximum of 6 hours, if your time of a journey exceeds 6 hours you already need two separate tickets.