I was looking at the difference between the ACT Clutch Release Bearing and the stock one. The ACT one is definietly beefier, but lacks the inner flats to prevent rotation that the stock one has. This bothers me a bit. Should it?

 

I wouldn't worry about it as long as the bearing gets installed correctly in the first place. My clutch jobs over the years have used aftermarket throwout bearings just like the one ACT sent you, and they never fell off .