(Getting serious for a moment...)
There have been numerous posts on EF ridiculing the Swiss rules for keeping pet animals - but I would ask y'all to take a moment to understand why those rules are there in the first place:
It is the duty of the pet owner to provide the animal with a life that is in keeping with it's species' basic natural and instinctive needs, within the context of your human environment.
The 'kept in groups/pairs' rules comes out of that.
In their natural state, some species of animals are solitary, these may be kept singly as that is what they prefer.
In their natural state, some species of animals are social. Forcing a naturally social animal to live alone, bereft of the companionship, security, stability it derives from living with conspecifics, is actually quite cruel. You are depriving the animal of something absolutely fundamental to it's being. Hence, the law says these animals must be kept with others of their species.
The BLV website discusses what is required for many common pet species. Look under 'Sozialkontakt' on the page for the species you are interested in. Here's the page for guinea pigs:
https://www.blv.admin.ch/blv/de/home...hweinchen.html
Now it should be said that there are always outliers. You may find an individual within a species defined as social that actually doesn't seem happy living with conspecifics.
If you run into a situation where, after trying to meet the requirements for a social species, it becomes clear that your individual animal is unhappy living socially, an exception to the rule can be obtained. Afterall, the goal of animal welfare legislation is to provide for the welfare of the individual as well as the species. In such cases, start by speaking to your vet.
Key is to understand what your pet needs in order to live a happy artgerecht life, and to provide it.
/sermon