A) he won the ball quite cleanly without touching Aguero at all, so yes, it was a good decision if that's the criteria against which we judge any tackle.
B) he's been perfectly fine this season, and had a fine game last night, especially in the first half when he nullified Silva's impact significantly.
C) in his 8 full seasons of senior football prior to joining us, he's averaged 37 games a season, which given that he's undoubtedly missed a few due to suspensions is a pretty good fitness record I'd say.