since there is no crosshair option in operators, would it be fair to use an app like crossover to have an external crosshair in the center of the screen?

Imo there are a lot of games that don't have crosshairs by default, I would say no as it would give an advantage to players on where to aim better , unless you do the F9 Console and minimize it to show your cursor and that can be used as your "crosshair" since its a game function within the Roblox client

If FPS Unlockers are allowed then I don't see why an external crosshair wouldn't be allowed either.

It's whatever. You can use one if you want to.