Rabbits have strong back feet, they can and will kick a guinea pig even if they go to hop away so yes it can kill a guinea pig.
Its not generally ever safe to put 2 different animals together.
I'd personally put a guinea pig and a rat together before I'd put a rabbit and a guinea pig together, all animals can be unpredictable at some point in their life so if you think you've had your guinea pig for a year and it just CAN'T hurt anything because its so sweet, think again, that's how "accidents" happen and "accidents" turn into deaths very quickly.
Learn how to spell guinea pig though, its guinea not ginnie or ginie, not being picky its just the point if you own one at the very least you should know how to spell what type of animal it is.
Good luck, Google it, not all sites can be trusted but you learn over time, in order to learn you need to have trial and error, make mistakes and learn from them, don't make the same mistake twice, that's how I learned, its the best way, research as much as you can and if you make a mistake and it turns out in death, start over again and do it even better.
Agreed do NOT listen to pet stores, most employees are there for a PAY CHECK NOT for the animals. Pet store animals are not healthy generally, they have genetic diseases.
Yes they eat different things there's absolutely NO safe way to house them together, side by side yes, but not in the same cage.
Get 2 rabbits and 2 guinea pigs if you want cage mates, its the safest way.
Check out www.chins-n-quills.com the Other Pets section and they will help you out and let you know right from wrong, they know best.
EDIT** When I meant put a rat with a guinea pig I meant for 5 minutes in a proof safe living room supervised, its unlikely that I would even do THAT.
I would not let any animal of DIFFERENT species live together, its wrong and you'll find out the hard way that they will end up dead.