What are pivot brakes?


Caliper brakes are available as single- or double/dual pivot brakes. Is that what you mean?


Please invest some effort into using the right words. It makes it easier to provide useful answers.


Front and rear brakes are completely independent systems.


As long as both are functionally OK by themselves, there are no problems with mixing systems.


Can become difficult to find levers that match both each other, and the different brake types.


Front and rear (firm) braking affects the bike differently anyhow, so the "feel" will be different no matter what you do.
Any rider who brakes more accordingly what happens to the bike than what happens at the lever will do fine.





I asssume you mean "should the fork fit".



I assume you mean "replace headset bearing cups too".



Why are you talking about stems?


If you want to use another fork, the steerer tube diameter is what decides if the fork will fit the frame.


But of course, the steerer tube decides what stem you can use in/on that fork.