I honestly don't believe you would have to trade Rondo if you signed Williams. You might have to for ego and personality reasons, but I don't think you would for basketball reasons. Williams a fairly big PG at 6'3" 210 pounds. He is also pretty darn strong and an excellent shooter. He would struggle with the size of SG's like Kobe, but I think he'd be fine against most SG's given his build. As an example, Bradley is only 6'2" 180 pounds and while he is more athletic than Williams, he isn't nearly as strong as Williams is, and people around here are fine with Bradley starting at the 2 guard spot.
That said, it would seemingly make more sense to go with Williams and Bradley as the starting back court and trade Rondo for a big man (like a Josh Smith, Al Jefferson, etc.), who while maybe not as good as Rondo, would fit a lot better with the needs of the team.