High-speed trains usually have mandatory reservations yes. Florence to Milan isn't a short ride for European standards btw : 300 km distance in under 2 hours.
Your Pass is for 2nd class so you can't buy seat reservations for 1st class. They often cost the same but then your pass wouldn't be valid.
- Florence - Milan is 2x10€ yes
- Milan - Lugano is easily done with the hourly RegioExpress train : reservation-free, just hop on and find a seat. (no need to buy 2x11€ reservations for the EC)
Reservations on regional trains are impossible and reservations in Switzerland are a waste of money in general : trains are frequent and have lots of seating. Nobody does them anyway. I've lived here all my life and never bought reservations once, it never occured to me :)
Too bad that you didn't get seats together, you could always gently ask the passengers on-board the train to move if possible.
Edit : Nobody will have reservations on the regional train Milan - Lugano as they're not available ! Simply board the train early and you'll find seats together for sure