Vets don't always know everything

especially about rabbits and guinea pigs and anything that's not a dog or cat or cow . . .
Rabbits and guinea pigs are really happier with their own kind, because they can communicate properly.
Lots of people have different species together, that could harm each other, but usually don't, like dogs and cats, chickens and ducks, cows and sheep or whatever. I think the only proper way to keep rabbits and guinea pigs together well would be to have a huge huge enclosure, with loads of places for them to hide away from each other, and have two of each species so they have company, and feed them separately. But then you still never know if one could attack the other or accidentally hurt the other.
But then again, people let their dogs and cats spend time around guinea pigs and rabbits, if they're used to them and sure they won't hurt them, which is pretty similar. So I think that if you know they're fine together and they've loads of space, then they'd probably usually be ok around each other. There is still a risk though.
Like my dogs and cats love each other but my old dog accidentally hurt a kitten and it had to be put to sleep

But I wouldn't separate my dogs and cats because of that.
So what I mean is, sometimes it could work fine. But it is easier to just keep them separate really.