A couple of comments.
The FO was NOT content with the OLine last year! That's why they dumped 60% of it(Nails, Ruddy, and Perry) and let 20% (Wade) walk away with a bloated contract from Houston. They simply couldn't do anything about it last year. They made some guesses, Dixon will get well, Nails will be able to come back, etc. that didn't work and were stuck for the season. The changes made this year in Free Agency, James, St. Clair, and McIntosh as well as the draft, Carey, Hadnut, and Pape, tell us the front office is doing what they can to fix the problem.
Now back to the question of the thread, center. McKinney is an unknown to the fans and yes, he does represent a guess and a wish by the staff but that's the way sports goes especially football in the free agency era. What is our depth? Long term, I would expect St. Clair would be moved to center where he had a similar, stellar college career to McKinney. Short term, if St. Clair is playing RT you could see Billy Ray Yates or Rex Hadnut snapping the ball. Both will be getting a chance in camp to see if they deserve roster spots. If the injury bug strikes then Ruddy is still out there but I would hope that would be a last, last, last resort.
Best case scenario for our Oline would be, imo, Smith, James, McKinney, Jerman, & Carey winning their spots and starting with McIntosh backing up both tackles and St. Clair backing up the three interior line spots. That would be my dream line-up, not just for this year but for four or five years out, which would be grand!