Not really seen the Welbeck challenge from a decent angle as of yet so it's hard to make a judgment on that, but I honestly think the Rooney challenge would have been a harsh sending off.
Yellow card, yeah, about right.
His feet were low, it wasn't over the ball, it wasn't what you'd call excessive force and his second foot followed through pointing studs downwards. Not too dissimilar from a few of Kompany's challenges, that said Komp's has been sent off for them in the past so I've definitely seen them given as a red.
In a derby though, as a ref, I would ere on the side of caution and give the benefit of a doubt. Shame that Dean sort of lost it towards the end, handing out cards left, right and centre. I actually thought Rooney was lucky not to get sent off for dissent at one point.