Guinea pigs are social creatures, you cannot keep them alone. Single piglets are lonely, the lack of company weakens their immune system and mental health. The key to their happiness is one or more guinea pigs to live with. No human or other species can fill that gap in their life. Cavies are together in 24 hours a day and they provide company for each other.

It is strictly forbidden to keep them with other species. Predators can kill or hurt them, their presence or scent are constant source of stress for the cavies. Bunnies, even dwarf rabbits, can easily cause deadly wounds by accident, and they can infect (and even kill!) each other with their own microbes.

Some cavies seem to be loners, appearantly they like no other guinea pigs. When it happens in a row, the reason behind could be traced back to early socialization, but not liking the first 3-4 options is perfectly normal.
Persistance could be the key, even the pickiest animals can find their mate when you provide enough options for them. To reduce the risks, let them meet first before commiting to a new animal.

Cavy combinations

Boars

Boars can be kept in pairs with certain precautions. Three or more boars rarely get on, better not risk. Adult males are harder to bond, so the general method is to pick an adult and a baby boar. When you choose this combination, there should be no tensions until the smaller reach adulthood, the hierarchy is rather obvious. Later on there could be problems, but if they can deal with them, peace should be permanent.
The other method is to choose two brothers. Adulthood could be problematic, but they can get over it, too.

To improve your chances, boars must be kept in larger places. The minimum size is at least 4x2 grid cage, but the bigger the better. Make sure they have two of every accessories: houses, water bottles, bowls, etc. Their houses should have more than one entrances so noone could be cornered. Give them the opportunity to avoid each other if that's what they want, so the houses should not face each other. If their home is interesting, they are less frustrated, and that's why they are less likely to bully the other. Make sure they have the chance to run outside of the cage every day. It's even better if they have a permanent run.
Our behaviour can also help. Always serve the dominant cavy first. Also, if you get your animals from a decent breeder, ask two animals that fits, a dominant and a submissive one.

Sows

Keeping sows together barely cause any troubles, if they have a proper place to live. Their number could be as high as you want, but make sure their cage is large enough. A neutered boar can be added to their group.

Opposite sex pair

It is a very harmonious combination, the chance of potential problems are extremely low. However, it causes a large number of offsprings, and lots of pregnancy related medical issues. Leave breeding to the professionals, so the boar must be neutered. (Neutering sows are very dangerous, vets do that only when there's a medical reason.)
If your place is large enough, you can add one or more extra sows to the group. Aware that you cannot add any more boars, they cannot live together when sows are present.

Pair or group?

If you have boars or don't have enough space, pair is the only option. However, if you want sows or sows with a neutered male, we suggest to keep a group of at least 4-5 cavies.
Guinea pigs naturally live in groups, it is their normal environment that gives them the feeling of safety.  They are prey animals, the larger number means more ears and eyes to listen. When you have a group, your animals become more relaxed and confident, they have every chance to detect predators in time.
When they live in groups, they can find their favourite company. It is enough to tolerate the rest of them, but they can choose the one who they can spend most of their time with.
Group is always more stimulating, something is always going on, it's more difficult to get bored.
And eventually, the view of a group of cavies is always alluring, you can watch them for hours as they live their lives.

On the other hand, when you keep more cavies, it means more responsibility as well. Medical bills could go up to several hundreds of euros. The more guinea pigs you have the more bills you can expect. 

But, if you are financially stable and have the room, we suggest to keep a group.

How to bond cavies

When you bring a new animal home, keep it under quarantine for two weeks. It is crucial, otherwise it can infect your existing piglets. During this period you must keep the new one in a different room and always wash your hands after dealing with it. Change clothes as well, if it was in contact with the new piggie.
When you buy your piglets from a respectable breeder, you can ignore this, but take it extremely seriously when the guinea pig is from a pet shop or any other questionable sources.

Introduce them in a neutral ground. It should be scent-free. Use some hay to distract them. Make sure you have a towel in your hand, because there is a chance of a fight. Their incisors are very long, believe me, you don't want to separate them with bare hands.

In an ideal case cavies are happy to see each other and show no signs of aggression. In a slightly less ideal situation, they might clatter their teeth,  rumbling and dancing, chasing each other or "fight" by practising the head holding ritual – whoever drops their head first are dominated. If not both of them want to be the leader, this can be resolved quickly. The dominant generally mounts the other, the submissive accepts its own role and they live happily ever after.
Even when they both have this kind of ambitions, the story can end quickly, if one of them is significantly stronger.
Problems can occur when they are about the same size and none of them wants to surrender. When this is the case, there is a good chance that they can't be kept together. 

Once they spend an hour together in peace, you can take them to their final place. It should be cleaned and scent-free. If you have more than one existing cavies, the newcomer should be introduced to the dominant piglet first. Once they are happy together, let the others know the new one.