- How early I need to reach Station/platform to board these?
These are trains, not planes. A couple of minutes before departure should be fine. Note that in Roma Termini and Milan (and some other stations) there are now access checks to the platforms, but that costs maybe 1min extra in my experience. Being at the station 15mins before departure should be fine. The train platform typically only gets displayed some 20-30min before departure anyway.
- Do I still need to get these tickets stamped (…)?
No, you have to convalidate (i.e. stamp) regional train tickets that are not train bound. If you have a PNR you don’t have to stamp (it might be difficult to fit your print-out into the convalidator anyway). In fact it suffices to have the PNR, I often just show my cell phone with the confirmation SMS showing the PNR or the conductor just asks you for a random digit of the PNR (assuming you are seated at the assigned seat). You may be required to show something (SMS or printout) at the above mentioned platform access checks though.
Source: Personal Experience.
Edit: These rules are for the Italian trains (Trenitalia and Italotreno alike), for the train to Paris (which seems to be operated by the French) rules might be different and I would carry a print-out to be on the safe side.
Credit:stackoverflow.com‘