Hey :)
Did you have a look at my answer in your other thread?
Eurail is valid on high-speed trains of course. However depending on the country/company, you need to add seat reservations (extra costs). Some routes are more expensive than others, esp. international ones.
Rail Planner is missing a lot of trains for July. For the most part, the railway companies themselves haven't published the times. As I suggested, look at a random April day.
There are no night trains on the routes you mentioned. In general, they're very rare in Western Europe (killed by low-cost flights and high-speed trains). The more east you go, the better it gets.
Here's a map: https://www.seat61.com/european-train-travel.htm
Some helpful links:
https://www.seat61.com/european-train-travel.htm
https://www.seat61.com/how-to-use-a-eurail-pass.htm
https://www.seat61.com/interrail-and-eurail-reservations.htm
This website is our reference. You can even enter Venice-Barcelona and it will suggest convenient options (not necessarily the fastest as most people don't want to spend 16 hours straight on trains).
EDIT: about that Venice-Barcelona journey in particular -> Google Maps might suggest overnight Flixbus or similar (you do not want that). As you probably can't extend the trip, please take a 1h30 flight or skip either city. Otherwise to put it bluntly, you'll spend the whole trip in transit.
