Guinea pigs and other pets
Introducing your guinea pig with other pets
Bunnies
In reality, bunnies are much bigger than guinea pigs, and they have strong back legs that they can kick guinea pigs with. If you want to take the risks, I'd advise introducing the bunny and the guinea pig when they are very little. That way, it will be easier for them to bond and get to know each other better. To introduce them, put the guinea pig in the cage first, then once it's scent is in the cage, place the bunny in the cage with the guinea pig. Now there is one VERY important thing you must know. Bunnies can kick guinea pigs if they feel threatened in any way, so this whole process is a bit risky. For the first 2 or 3 weeks, keep a sharp eye on both the bunny and the guinea pig. If any fighting occurs, separate both animals IMMEDIATELY! Guinea pigs won't fight back because they are genital, so if the bunny looks like it's going to hurt the guinea pig, separate them and don't put them back together ever again! If all turns out alright, the bunny and the guinea pig will live harmoniously, and they will be kind to one another. Don't keep them in the same cage, because anything could happen bad anytime.
Cats
Cats are big and scary to guinea pigs. Introducing the two total opposites will be difficult. Firstly, you MUST ALWAYS hold your guinea pig close to your chest. The cat will most likely not choose to hurt the guinea pig, but anything unexpected could happen. Carefully and cautiously bring forth the guinea pig to the cats nose, and let the cat sniff the guinea pig. When you choose to ever introduce the two animals, always hold the guinea pig, and have a friend or family member hold the cat. That way the cat won't be able to pounce.
Rats, hamsters, and mice
When introducing any other type of animal to the guinea pig, you must always do it with caution, and supervision. Rats may get along okay with guinea pigs, but rats tend to be a little bit nippy when meeting other animals. Also, rats play fight for fun, and guinea pigs never play fight, so if you see your guinea pig bleeding, or you see the two of them getting along badly, separate immediately! Same with mice and hamsters.