Hi Teri
No, seat reservations are not possible on IR trains.
Seat reservations on Swiss trains are optional and only possible on IC trains. For border-crossing EC trains as well as PE (panoramic) trains, seat reservations are mandatory, though.
Swiss trains are so to speak mass transit; hence, you do not have to prebook a specific train; you simply get on a train that suits your travel plans and take a free seat available on the train.