now i know this is a biggy, but please bear with me...
i know your heart is in the right place and you have good intentions, but please try a local rescue centre,
as the others have said (& i hadn't really considered it in the supermarket bargain basement sense before, but very true) the legitamate rescues are not for profit, they only try & cover their costs through donations or fees. They work tirelessly for the love of the animals in their care, usually reaping no reward but the satisfaction of happy , healthy, cared for pigs.
They always have after rehoming backup care & some help with nail trimming, dental problems, etc. They usually have background info if available of any health issues & recommendations on how to sort health problems that may reoccour.
Most Rehoming centres have poorly pigs that they are trying to nurse back to health, and past experience to prevent worsening or spreading of illness diseases. Most pet shops just try & sell them so that they are not their problem & they dont have to pay to treat them.
Or in supermarket terms, you dont just buy the ingredients. the rescue provides you with the ingredients, recipe, how to prepare, store, cook & reveal the creme de la creme of guinea pig ownership.
How about contacting one of the local rescues on here or try this link
http://www.theguineapigforum.co.uk/forumdisplay.php?19-Rescue-amp-Rehoming

perhaps you could foster for a proper rescue centre, just to give you & you're mum an idea what you are letting yourself in for?
Special needs Piggies are hard work... very rewarding.. but definately hard work. this may also give the rescue centre a bit of a hand, as your using your time concentrating on the foster piggies, rather than the rescue centre. You would also have the support if needed close at hand.
I am relatively new to this forum, but there seems to be a knowledgable, network of experience on here that i am sure would guide you in the right direction if you were to rehome special needs piggies, but as SuzyGPR suggests she has to constantly mop up after P@H.
'P@H = profit making piggy production line pigs' half the staff probably have no idea on aftercare of these wonderful little creatures & no idea of diseases, symptoms, medications, housing, feeding & nutrition of them. i wonder how many pet stores know about the consequences of roan x roan breeding/ lethals, etc? but many of these proper rescue centres, know the problems & often have affected piggies to look after.
Please don't fall into the pet shop breeding trap of any store. I'm sure there is a link here for rescue centres, the chances are there is one locally to you.
?/ If one of the other members could post a rescue centre link, please have a look.
Good luck in finding proper rescue pigs, they need all the loving homes they can find.