I'm not a rabbit keeper (never had any) but I know their food differs in some ways.
Rabbit pellets have a different vit C content (GPs have Vit C supplement). They only have an eggcup full whereas I give the guineas a handful a day (so that they always have some left over)
Rabbits main food is grass & hay, they have to be careful with veg.
Fostered animals would be a mixed bag- you might get some that are waiting to be neutered or waiting till they're old enough to neuter.
You might get some unsocialised , unfriendly (so would be difficult to cuddle).
Rabbit accomodation is huge compared to GPs and they need more outdoor access. Do you have space for outdoor runs- grass, patio area.It needs to be secure, these are someone else's animals so you definately don't want them making a bid for freedom 
You need to be careful WRT keeping the bunnies and guineas apart- alongside the feed difference and the risk of rabbits harming guineas, there's a respiratory bacteria Bordetella Bronchiseptica which can pass from rabbits to GPs.
The rabbit rescue will give you guidelines to the sizes of accomodation and quarentining new animals.
Will they lend you cages/hutches/runs or do you need to supply them.
Find out how you are to decontaminate between visitors.
Who pays for food?
Vet Bills (I'm assuming they will pay for neutering and vaccines, claw and dental care. But what if your Foster Furries have an accident or illness?)
What if you get attatched to them? Can you switch off and return them when it's time for their Furever Home?
Why rabbits and not guineas? There are just as many pigs in care. And you already know pigs and are geared up to their needs.
I bet the Rescue you are involved with will have people surrendering GPs or asking where they can do so.
Good Luck. I'd imagine alot of Rescues wouldn;'t function without fosters.