I don't think it would be a good idea. They are different species and have totally different social skills, not to mention that most hamsters are solitary and probably wouldn't know how to play. Also the hamster might feel threatened by the guinea pig's size and attack him.
It might be fine -- this time, but it's generally not a good habit to get into. Aside from the potential for injury to both animals, it's very stressful for them to meet new animals, so it wouldn't really be a "playdate" as we think of them; it would be an unnecessarily stressful event that they have to endure for, really, no good reason.