I wouldn't even let them play together, rabbits have such powerful back legs that one kick and you have a dead/fatally injured pig, even a small rabbit can cause a fatal injury. Also rabbits can carry Bordetella which does not affect them but is fatal to pigs.

Also you MUST get 2 pigs, they need companionship, getting only one simply is not fair.

If you are wanting a playmate for the rabbit why not get him another rabbit?

Is your rabbit neutered? I ask because if he mounts the guinea pig he could badly damage the pigs back.