Ok - no rabbit experience here but I've had pigs from when I was 9yo and I've passed the mantle to DD (but she chose some boars so I've had to learn a whole new set of Piggie Rules) 
GPs need hay 24/7. They shouldn't really have straw (it can be sharp, they can damage themselves, and it has minimal nutrient) but the past few years the hay has been coarse. I use soft barley straw in winter at the back of the haybox.
They have the 'trickle feed' digestion that needs to be fairly constantly grazing (then at dusk they start to pooh eat to extract the nutrients - nice)
Grass alone or veg would give them loose poohs, they need the roughage (also keeps their teeth down and stops them getting bored and plotting) They can get gut stasis and liver failure if they go without food for 7 hours.
There's lots of bedding choices, I sometimes use shredded paper indoors and give hay cookies (solid hay disc) but mine prefer hay.
Rabbits aren't considered to be good childrens pets.
Guineas are lovely and snuggly but some people find them a bit dense and boring.
Rabbits and Pigs together - no.
Their diet is different , a rabbit can give a GP a respitatory virus, they can kick the GP causing injury/death, and they speak a different 'language'
With GPs you need 2+ but you don't need to innoculate or neuter (same sex pairs)
Size of accommodation - mine have a nightcage for winter (4'x2' which they decided they would not tolerate - we have a new boy after one died last year. The original ones were judgey but tolerant)
They were bought an indoor run (over 3'x5' ) last winter and have a shed (6' square approx) for winter days and the warmer months.
With a heater
And a fan
And a light
And a removeable but reinforced window (mesh and bars)
Oh yes.