Is it OK to replace a 750 ohm 5W ceramic resistor with a 750 ohm 10W ceramic resistor
Is it OK to replace a 750 ohm 5W ceramic resistor with a 750 ohm 10W ceramic resistor?

Yes.

Will the 10W one generate more heat?

No.

Is it OK to mix one 10W resistor with three (existing) 5W resistors? (All 750 ohms and 5% tolerance.)

Yes.