Anyway, the Zandalari was a powerful nation, with many strong warriors and powerrful magics. That would be why the Mogu allied with them. Maybe they could have beaten them, but it would be a costly war.
But the Pandaren? Considering that they didn't discover how to harness chi or fight as unarmed monks until they started their revolution, and the Shado-Pan wouldn't come until even later than that, I think it's fair to assume that the Pandaren were probably living as peaceful farmers etc, and thus were ripe for enslavement.
There is still no evidence that the pandaren were created by the Mogu. And if they were like the Saurock and Grummles, why would that be an unrevealed secret when we've learned about the other races the Mogu made? The theory just doesn't hold water.