I have the same concerns as you. This summer the piggies are going to travel a lot. They are going to fly from here (US) to Paris, take the train to go the my grandparents where they will stay during the summer, then they are going to take the train (or car) for 4 hours to go back to Paris to then fly to Tunisia where we are mooving...
I am taking them with me in the plane, I would never put my piggies in the luggage compartment. Luckly small animals are alowed on carry-on on the plane I'm taking. Are you sure they don't allow piggies to travel with you ?
They should have a comfortable pet carrier (not the ones for guinea pigs sold at most pet store, they are usually too small...) but a small cat carrier would work, and don't forget to put plenty of fleece in there because it does get cold on a plane.
I heard that giving them plenty of cucumbers is good for them since they would get plenty of water from it...
Hope is helps !