no guarantee of seats without seat reservation

Any train related questions you can get a good answer on Seat61.com site  The owner of the site is an expert on worldwide train travel, especially for Europe train travel.  He even answers emails.

italotreno is private company.  They compete with the national company tranitalia.

Personally I would take italo train anytime if it is available than tranitalia.  Better train better service.  They also have a 2 to 10 group fares, it would be cheaper than buy 2 individual tickets for example.

Buy the first class tickets which when buy early is not that much more than the cheapest one but you have a better experience riding the train.

thetrainline is a 3rd party site to make your purchase a bit easier.