But why would you even take the chance, if you KNOW that it isn't what's best for the animals? You said that you KNEW that the rabbit could kill the pigs, and that their diets were different. WHY did you house them together?!
I used to let Smudge and Sunny play together before Sunny was bonded with our other pig. At first, they loved each other- Sunny sat there and groomed Smudge for the longest time. But, when Sunny decided that she didn't want to groom Smudge one day, Smudge got really upset and went after her and tried to bite her. I was watching them very closely so I caught Smudge in time, but he could have easily killed her. I haven't let them see each other much since.
But, the reason that Smudge went after Sunny is because in "bunny language", refusing to groom a dominant rabbit is a MAJOR insult. Poor little Sunny didn't understand this of course. She didn't think anything of her behavior, but Smudge was really upset over it. This is just one example of why they shouldn't live together.
And just because nothing bad happened to your GPs doesn't mean that housing them together is safe, you just got really lucky. It's like housing bearded dragons together- Some people's lizards will appear to get along just fine, while other's beardies will maim, kill, and eat each other. And of course, every one says "Oh, don't worry, I check them every day for injuries or signs of dominance." and then the next day, they have one or more dead animals whose deaths were TOTALLY preventable. It's just NOT worth the risk.