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My son has two pet rats - young boys - very friendly and calm. My daughter has a young male guinea pig - also friendly. We let them all meet on a lap and they seemed quite interested in one another. There was much sniffing and some "mouthing" but no one seemed to get upset or hurt. We didn't let it last long because we didn't know if it was OK.
No, it's not a good idea. Rats, while normally sweet, can inflict a serious bite. "Mouthing" is a bad sign. Even a cat may gently pat at a mouse before killing it. The rats are in danger as well since a scared, startled or hurt pig can also do great damage to them. There is also the possibility of disease transmission. Some species simply shouldn't interact because a disease that may like dormant in one could transmit, become active and possibly kill another. Please don't risk it.
If the majority of the feedback you are getting tells you it's not a good idea and has reasons and experience in why it should not be attempted then you really should listen to the voices of experience and not concentrate on one or two people who tell you it's possible.
I could go onto all of the forums and ask if hamburger is good for GPs. While the vast majority would tell me no, with good reasons why it's not, I would also get a couple of people telling me it is good for them but I doubt they would have reasons why or how hamburger is good for GPs. Just because a couple of people say it's okay certainly doesn't mean I should try it.