Foketi is not really the sort of centre I'm talking about in Walton at 13. That's no knock on him, but Walton is your more reliable, defensive player that helps connect the backline, something similar to what Stewart has offered at both the Reds and Force. I think that Foketi-Walton pairing is the way to go, particularly early on while Sua'ali'i is still finding his feet defensively. I'm sure he'll see time there through the season, but if he's there too permanently too soon, it really risks him being exposed and exploited by mature oposition attacks.
McKellar may get away with playing Sua'ali'i at 13 for the first couple of rounds though, and then switch him out before the games against tougher opposition come to really work his weaknesses.