We are going to Italy May 26-June 8th and going to these destinations Venice 2D, Florence 2D, Siena 2d side trip to Pisa, Cinque Terre 2D, and Rome for 4d. Are our plans too aggressive or just about right. We will take train between all the destinations
I think it's a little much, considering the short number of days, but if you're energetic go for it. You'll be spending some good time at train stations and on the trains, so you have even less time than you may think for sightseeing, but spending time in an Italian train station is also a nice contrast to everyday life, so it's hard to call it lost time.
Does the Pisa side trip come out of your 2 days in Siena? There's not much to do in Pisa except for a quick look at the tower. Perhaps work that in on the way to Cinque Terre taking a taxi to the tower and back to train if necessary. That would give more time in Siena.
Plan your Florence time well and get your museum tickets in advance so you don't waste a lot of precious time standing in line. Most things are closed on Monday. Do your homework and you'll do fine.
If you had an extra night in Venice and one in Florence, you'd be in better shape to see all the major sites there. Pisa is no more than a 1/2 day. But, if you need to move faster, it will be OK.
For me, there would be too much traveling. Assuming you leave the US on the 26th and depart Italy on the 8th, you have 12 nights. I find it easier to figure nights when doing planning. With that time frame, I would do 3 nights in Venice, 3 nights in Florence, 2 nights in Siena and 4 nights in Rome.