Lots of questions here - if you don't mind reading through my waffle I'll tell you some about my pigs .
I kept guineas from when I was 9-22 ( this was in the last century 
) Then my DD wanted A Pet ( she wanted a hamster because her friends had them, but I persuaded her to Think Pig )
Ours were all from Rescue , all adults apart from GP3
We started with two adult , solid , un castrated brothers .
They had our DC wooden playhouse (floor space was 6'6" x 6'6" and I could stand up in it ) DH did all the alterations-
2 windows could come out the gap was filled with bars, chicken wire and mesh to keep out predators and flies .
He put in light and a power switch for a fan or heater
They started with 3'x3' haybox made from an old cupboard laid flat (the door made the 'lid' ) They had deep hay beds .
Boars need a lot of space . We had an indoor 4x2 cage that they hated (for night in winter indoors) We ended up buying a pen , then C&C
C&C is good but the hay gets everywhere even with a liner
My pigs didn't like ramps , they liked ground level.
They had a big Bunny Business run that we could peg to the ground (84" x 48" and about mid thigh height ) They don't dig but other animals can dig in.
When one died we got piglet GP3 to bond (6 weeks)
Then we went through the cycle of one dies/add one
Bonding new pigs takes planning and we thought we cracked it ..until we got GP7/GP8 (sows) to add to GP6/GP4/GP5 ( neutered boar+ 2 sows)
It didn't work . We had to keep them in 2 groups ( their original) until we were down to the last 2 ( our boar and the less stroppy from the 2 sows ) They bonded and had a few nice weeks before he died .
So- they need protected from everything.
Damp , draughts , changes in temperture , too hot , too cold , damp grass
Over gorging on grass can give them bloat
Over eating dark greens can give them bladder stones
They need Vit ( in their pellets ) and food
They need hay
Predators -fox,cat,dog,badger,ferret,rats
Poisonous plants - they do tend to ignore but you need to check the lawn and don't give iceberg lettuce,onions,any bulb,potato, too much fruit
They are not the sharpest tools in the shed and very defenceless
They don't jump high or kick. They don;t scratch . They cannot turn their heads quickly .
But for all that , even with rodent teeth they really don;t bite unless they are pushed to it . You have to give a guinea pig reason to bite (unless it was my GP5 , she would give a 'gnaw' if she saw fit)
Boars or sows?
My DD wanted a boar so we started with the 2 boys .
Boars are tidier ( my sows were messy ) but don;t smell like many other male rodents .
Neutering won't change them except make them sterile. (And reduce risk of impaction )
A lot of Rescues will castrate now to make re homing of boars easier .
Our pigs had the small bedroom (no door on it and I put the radiator off) in winter . They liked to ignore water until 3am . They liked to chew cardboard at 4am.
They are active dawn/dusk . They eat like locusts then when its quiet eat their first pooh (very discretely)
They are cuddly and in time will give you a warning when they want to pee (moving, stamping their feet . You;ll miss the forst few warnings then learn )
They have quite specific health needs , they go Well-to-Dead very quickly .
You'll need to keep their claws trim they don;t blunt them walking .
When we had our two groups I had a rabbit hutch (legs cut off) it was a bit bigger than a guinea pig hutch , it was inside the shed . The sleeping area isn't big . We had it open (door cut out) removed the inner wall and filled it up with hay .
They went through loads of hay , newspaper . Some people like fleece .
I used it on the floor of the C&C and a cardboard box of hay to sleep in.
Washed in a PetBag to keep the hay from ruining the washine machine debris
They are lovely. Cuddly.Gentle.Noisy.Messy.Greedy.
I do miss ours but they are hard work