I haven't actually taken my cavies on an airplane. My advice is based on years of working for an airline and logging lots of travel miles. You are right......Frontier is just about the only domestic airline that will permit guinea pigs in the cabin.......because they're rodents! Never mind the "support animals" that are sometimes permitted aboard. I know a woman who raises cuys and she tells the personnel at the security checkpoint that they are cats! Hmmm.....cats sans tails?
Keep in mind that whatever carrier you select absolutely MUST fit under the seat in front of you. That takes away your leg room and you should plan to ship all your luggage in the cargo hold. The type of carrier isn't so important as if you were going to ship your girls in the luggage compartment. If shipped that way, most airlines have pretty specific guidelines as to size, strength, and construction. I think that you would be best served by a soft-sided sturdy carrier to reduce weight. If it is soft-sided, then you should be able to push it under the seat even if it is a tad too large. There should be guidelines on Frontier's website.
Keep in mind that the floor is the coolest area and that the girls should have a cozy of two just in case they get too cool. You may want to put some hay in the carrier, but not a water bottle because the water will leak all over everything. Start working with the girls now to teach them to drink out of a water bottle when you are holding it. That way you can occasionally offer them water during the flight, although they may not want it. My boys rarely eat or drink during a long car ride and I doubt that they would be any different on an airplane.
I wouldn't worry so much about the noise as they will be under the seat and the ambient sounds will likely be muffled. They have each other for comfort and you can occasionally reach in the carrier for a reassuring pat. So long as they know that you are nearby, they shouldn't be overly stressed. Insofar as preparing them, I can't think of anything other than to feed them well and allow time for toilet needs just prior to leaving for the airport.
All the best with your move! I'm sure the girls will be just fine.
