For Salou to Venice, I'm showing a train trip of 20.0hrs-31.0hrs depending on the run. Two runs go through France and into Switzerland. One run (the 20.0hr run) goes through SE France in into Italy. Several train changes along the way. All 3 runs that I see have night train segments.
You certainly don't need a rail pass of any type for your travel in Italy. Aside from the fact that P2P tickets are less expensive, Italy charges a seat reservation fee to railpass holders for the ICP and Eurostar-category trains. For the ICP trains, it's 3Euro. For the ES trains, it's 18Euro. Again, this is on top of the cost of the pass. The train runs between Venice and Rome are quite frequent. You can easily buy them when you arrive in Venice.
For Salou the Venice, I would suggest flying from Barcelona to Venice on www.vueling.com. I'm looking at 12/8 and I see a flight departing at noon and arriving at 13:50. Cost is 50Euro. However, these inexpensive airlines charge fees for everything so I would expect this cost to rise to about 80E-90E. Still, 2hrs in the air really beats 24hrs on the train. Plus, you don't need a railpass.
As a side note, you don't need a railpass to buy a P2P ticket on Raileurope. The website suggests that if you are traveling between countries, you might want a railpass. But, you really don't need one.
