People are watching and we know who is corrupt," he said. "If Najib as prime minister choose them as ministers, his political party will lose the next general elections."
I could go either way on is/are. I can see why are would be correct, but I would probably say 'is' myself, as if I were looking at each potential minister individually on a case by case basis, rather than at all of them as a group.
But two words you defiitely need to change in your next sentence are 'choose 'and 'elections'.
It should read: If Najib, as prime minister ,chooses
them as ministers, his political party will lose the next general election