Condenser Motor

In principle, yes electrically, though the arrangement will be less energy-efficient. The larger motor will be doing a smaller percentage of its available output. Starting currents will be higher, and this needs to be taken into account in the design and selection of the motor overload protection device, the power circuit protection device and the power circuit conductor sizes.

Mechanically, why is there a difference in speeds, and does this matter to what is being driven?

You could not if it were vice versa. But as it is, you can simply put a restriction to the air flow to avoid overcondensing, if necessary.

Are you using the same fan blade, moving it from the old motor to the new? (If it is the same blade, 580 (960/900)³ ≈ 704 W < 750 , so this aspect looks okay.)

If the new motor will use a different blade, you should compare the air flow of the old and new combinations; the new one should have equal or a bit greater flow.

The difference between 400 and 415 volts may be within the permissible range of either motor, but you might want to check that, too.