I see that my train from Florence to Venice arrives at Santa Lucia but so far the schedule shows trains leaving from Venice to Rome only from Mestre. Does it really matter which one I arrive/leave from?
If you type in a starting station of "Venezia Santa Lucia" you can see the trains departing from that station. I've only ever used the Santa Lucia station in Venice. It is the one on the island. The Mestre station is on the mainland. There is a shuttle train between the two stations. http://www.trenitalia.com/cms/v/index.jsp?vgnextoid=c79686605528a110VgnVCM1000003f16f90aRCRD
When I look at the schedule on Trenitalia, I see all the ES trains departing from Venice Santa Lucia (at 9:27, 10:27, 11:27, etc.) It says the origin is VE S.L., and then below that "Mestre" in red. It is confusing, but the train actually starts at Santa Lucia and stops in Mestre before continuing on to Padua, Bologna, etc.
Oh, that helps! Thanks!