I have placed small animals on airlines including rabbits, rats, and reptiles. Northwest was the best I worked with. An airline should have their pet cargo policy very clearly posted. Wintertime is an exceptionally bad time as is high summer when the airport you leave from is over 100F. When it comes to a decision whether your pet or someones luggage gets onto your airline, the luggage will be chosen over the pet and they will shunt the pet to another flight. They will not tell you if they do this. It is wise to consider a separate cargo service rather than just entrusting the plane you are on. Delta has their policies well written here:
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A cargo service is not cheap, it can come to $250 or more, but I trust it. Granted this is for a U.S. service, I am only using it as an example.
I recommend a cage in which you can attach a water bottle to the inside of the cage. Make sure it is attached firmly and as well, get a water bottle that can be attached to the outside of the cage so personnel can change it if needed. Using hay as bedding is a good idea as you can really pack it in. Just don't cover the ventilation grates. Orchard grass is soft. The pig will wind up pooping and peeing all over the place but there will still be plenty of hay to eat.
Mark the cage LIVE ANIMAL and strap all pertinent information including any vet info, your name and destination, the names of both someone at the originating point and someone at the destination who can recover the pig if you are not available.
Always be prepared for the possibility that airline personnel will deny your animal flight privileges. Have someone on hand who can take the animal or have some other arrangement handy. Call the airline before you go to inform them you are bringing an animal, ask them where you should bring it to and what you should have with you when you come.
This may seem excessive but taking precautions is a good idea.
As far as pressure goes, there will be no problem with your animal and the pressure controlled cargo area. The major concern is temperature and stress. If the kennel is large enough, a snuggle pack will give the guinea pig a good place to cuddle into. The hay will insulate the pig as well from temperature changes outside of the kennel.
Yes, there are plenty of scare stories of horrific airlines out there. 99 percent of the time, the owner was not prepared, the animal was not ready for transport (dehydrated, had a fever, etc) and there was little communication. Animals do wind up getting bumped off passenger flights and put into holding areas. I have never once had an employee mishandle my transport animals, shake them about, yell at them, etc. I have never lost an animal, even my preciously tempermental terrier mix or my delicate iguana. They can get ignored but not for longer than 4 hours usually. I think in a guinea pigs case, if they have the extra water bottle they are alright to be left alone though they may be stressed by barking dogs.
I hope this answers your questions. If you want a particular airline looked up, please let me know and I am happy to try to get specifics for you.