Having in mind that we are talking 3000rpm here / though some servos can reach as high as 6000, i dont think thats a design consideration. So i think they will work, assembly would be stronger but wear will be premature. In short i would use the normal angular contact bearings as they are more than strong enough for the job.

Size reduction is impossible as the nut should pass through, so that determines the size.

Size reduction in ball screw is absolutely irrelevant as normally you will buy the 3m ball screw as maximum shipping size from China and anyway you need to stretch the ball screw properly on the machine, so you dont need any gain in sizing there, see my second build from signature. As what you loose there from length is the gantry step spread, which is quite more bigger than the rotating nut assembly length.

So in short- you gain absolutely nothing


About the preloaded pair, you have the same in the normal angular bearing. But how do you fix that so it serves you? I see no way.