No offence to ilsonchick, but I've always assumed that rabbits and piggies shouldn't be kept together, and this snippet I found on a website seems to back my assumption up.
Whatever you may have heard, seen, or been told never, never, never keep a rabbit and guinea pig together. It will lead to misery for both, and possible injury or death for the guinea pig. There are so many reasons we could list for not keeping these animals together and we are happy to talk to you about this if you have concerns. In line with RSPCA guidelines, we will not rehome a rabbit and a guinea pig together.
Again, no offence to ilsonchick, she is afterall speaking from expereince of keeping both for some time, but I personally wouldn't run the risk if there's the slightest chance that the arrangement wouldn't work.