Hi and welcome
It is great that you want to do the best you can for your single, but also great that you are doing your research beforehand!
Please let your guinea pigs live alonside each other at first, so the new piggy can settle and they can get used to each other and make friends through the bars. I would strongly recommend that you plan for a permanent living alongside set-up in case any bonding is not coming off.
If you have access to a rescue or a place where you can date your guinea pig before bringing a new one home, your chances of success are higher but even then, things can still go wrong once they are sharing the cage and are working out the dominance.
If you do not have the dating option, then it is very important that you have an alternative option available in case there is no success. You can never predict how your piggies will react during bonding and the two weeks dominance phase afterwards.
Some piggies are never too old for a first companion (the oldest I know of was a 6 year old boar in Australia, who was brought by his new owners to find a mate via rescue dating and went happily home accompanied by another older rescue boy he had made friends with), but others that have been kept as singles from very young lose/never acquire the finer points of social interaction and at the worst lose the ability to perceive themselves as guinea pigs if they only have a human to bond with at the very young age when they depend on being socialised by the elders in their group. A very human orientated piggy may not bond or even be interested in other piggies.
The problem is that you can never know which way your piggy is until they actually meet other guinea pigs. Brace yourself that in any case they are more likely to be rather wary and can react over the top (fear-aggression) because of their insecurity.
You may find these guides here helpful in considering aspects:
What to check and look out for in new guinea pigs (vet checks, sexing, parasites&illness)
Sexing Guide
Bonding: Illustrated Dominance Behaviours And Dynamics
As to experiences of my own, here are two contrasting examples of two of my current adoptees:
My two year old adopted single Dylan (who spent the first two years of life stuck in a tiny cage and ended up with very weak back muscled from lack of movement) cherishes piggy company whereas my similar aged Beryn was obviously somebody very much loved pet before she was dumped on the grounds of a cricket ground. Despite being tried with a whole range of various options from groups, single neutered boars, single sows, baby sows (of which she was very literally scared stiff!) to the most submissive and gentle of neutered boars I couldn't find after months of search (Dylan), Beryn is just not happy with sharing her space with another piggy. She is now having a pen right between other cages, so she has lots of piggy interaction and a larger chunk of regular free-roaming time for her piggy interaction and stimulation although she still enjoys her human cuddles.
You may find these guides here helpful in considering aspects:
What to check and look out for in new guinea pigs (vet checks, sexing, parasites&illness)
Sexing Guide
Bonding: Illustrated Dominance Behaviours And Dynamics
Here is a video from Dylan's introduction my 4 year old sisters Morwenna and Mererid (the bonding sadly failed in the dominance phase when Morwenna would not accept that Dylan would punch higher in the hierarchy and Mererid sided with her sister). Initially it looked promising. The sisters had recently lost their beloved husboar/daddy boar Bedo.
... here from when Dylan was introduced to two 3 weeks old just separated baby boys to choose which one he would get on best (or at all):
Beryn:
How she is still profiting from interacting with other piggies through the bars:
Trying to get Beryn to accept Dylan by an (unsuccessful) series of cuddling sessions on the lap - as soon as she was free to show her feelings, she would make it clear that she didn't want to be with any other piggy, however friendly and patient!
PS: I am currently writing an in-depth article series for Guinea Pig Magazine about single guinea pigs in all their forms and the challenges and problems that come with keeping them and - if possible - pairing them up. The first part is going to appear in the January edition and the second part will be published in March if you are interested. GPM is available worldwide as a download option.
Guinea Pig Mag
