Not having a reservation on a train doesn't mean you wont be able to board the train. It just means that you will probably not be able to get a seat on the train and you will have to spend your trip either standing or sitting on the floor between train wagons. I once spent 4 hours sitting on the floor between wagons traveling from Hanover to Nuernburg because I didn't buy a reservation.
As you said, there are some trains that do require reservations, such as the TGV or the ICE Sprinter. If you board a mandatory reservation train without one, you'll be fined on the train.
As long as both of you are not citizens of any European country, Russia, Morocco or Turkey, I would suggest looking into buying a Eurail three country select pass for France - Switzerland - Italy. With two people travelling together, the saver pass is a good option. Or if both of you are under 26, you can get a youth pass for a better price. There's more info at
www.eurail.com