Hi and welcome! Good that you are doing your research and are asking first!
The rescues we recommend have all:
- a mandatory quarantine and any necessary health care in rescue. Only perfectly healthy piggies will be listed for adoption..
- any incoming sows undergo a mandatory 10 weeks pregnancy watch.
- any recue born babies are correctly sexed, separated at the appropriate age (3 weeks for boars as the earliest documented sibling pregnancy is 24 days). Have a look through our pregnancy section to see that unplanned shop and breeder or shelter pregnancies are anything but rare and can end in tragedy and/or major headaches for people who do not have the space and capacities to deal with one or more large litters.
Rescue born babies are only being rehomed when they have reached a good weight that prevents them from falling ill and dying as easily as stressed out shop piggies that are sadly often coming down with highly infective ringworm (fungal), mites or respiratory infection. You can find a pretty steady stream of cases in our Health and Illness section.
Young piggy babies may be very cute, but they are also a lot more fragile. You have babies only for a few weeks, but adults that live to 4-7 years on average.
- any rescue guinea pigs have been carefully bonded for character compatibility, so that you do not have to worry about fights and fall-outs like when your cute little shop bought baby boars hit the teenage hormones together between 4-14 months old. You also have the rescue to fall back on if problems happen along the way for whole life of the rescue piggies.
There is no license or control of minimal standards, so anybody can call themselves rescue or breeder, and the conditions and results are all too often correspondingly dire - and quite often not obvious for the unwary buyer who can't spot the subtle warning signs.
Please be also aware that pets@home import their piggies from rodent farms on the Continent. During the several days lasting shipment, the guinea pigs are not gender separated, even if they are so in the shop. I was caught out once that way, and to my knowledge, the practice hasn't changed since.

How well boars get on often only becomes obvious once they develop their adult identities. the key is that they are character compatible; finding the right pairing to create a stable bond that often profits from an age difference takes time and experience - something that any places that are only interested in the money but not the welfare and future happiness of their animals can't and won't provide. Our forum is littered with shop or breeder bought boars struggling when they hit the big hormones and have to be separated. Have a look for yourself!

You also have to be aware that shop or breeder guinea pigs have usually not had much in the way of human contact. They are prey animals and not born as cuddly pets.
http://www.theguineapigforum.co.uk/threads/boars-sows-or-mixed-pairs-babies-or-adults.108944/
http://www.theguineapigforum.co.uk/...stincts-and-speak-piggy-body-language.117031/
http://www.theguineapigforum.co.uk/pages/guinea-pig-rescue-locator/