Certainly I think that GSP could KO Rickson, as odds are he wouldn't get taken down. But Rickson is much much better than you give him credit for. There is no reason to think that a guy who he would have a noticable size advantage over would control Rickson on the ground. Who from all known reports is basically a demon on the ground. Any of the grappling I have seen him do has been top notch. Also comparing Rickson to BJ.... For arguments sake lets assume they have equal BJJ skill, If Rickson had that omoplata in deep then he would have 30lbs of muscle and therefore strength on BJ to force it in. GSP is not an eliete guard destroying machine, hell I would favour BJ if he was the same size.
That is a real possibility. I don't think that Rickson could score a takedown on GSP. Though I believe Ricskson would start pulling guard when this realization sits in. I don't believe Rickson would want a restart. The fight may become very dull while GSP is strong, and Rickson would be trying to wear him down. I think it would be more likely that Gracie would submit GSP than GSP would bust Rickson up on the ground. Stand-ups and restarts would be likely otherwise, and then a GSP decision would probably occur under today's rules.