This is a very simple one: Don't mix guinea pigs with other animals. Even if your dogs and cats or rabbits are the friendliest pets on the planet, accidents can easily happen. Bunnies can kick, and cats and dogs... well, let's face it, they're great big predators red in tooth and claw as far as the instincts of your poor guinea pig are concerned!
Guinea Pigs and dogs - not a marriage made in heaven...
All animals act on instinct, even if they’re just trying to play, and that can spell trouble when one of the animals is more fragile than the other. Guinea pigs can trigger hunter instincts in cats and dogs, and even a paw placed on a wriggling GP can cause injury, not to mention extreme stress. Rabbits, meanwhile, can't understand GP body language and may get a bit too rough during play. The same applies to other pets, including rats, chinchillas and gerbils: they are all different creatures with different needs, and simply won't get along.
If you're still keen to introduce your pets to each other, you’ll have to be present at all times, holding back the larger/carnivorous side of the equation. Keep the meetings brief, and supervise the interaction. Getting a dog or cat used to the sight, smell, sound and movement of a guinea pig is useful, as it will prevent the larger animal getting excited and potentially dangerous every time it senses the other pet.