Installing new bathroom heater/fan combo which requires a dedicated 20amp circuit. I got that 12/2 wire ran direct to the breaker.... the problem is the fan that was existing had a switch that I was planning on reusing for this new one. Only it is 14/2 wire, is it ok to run from the junction box in the new heater/fan the main power which will be the 12/2 AND the 14/2 for the existing switch?
This is allowable, but since there is any 14 AWG wire, it is a 15A circuit.
That means you must use a 15A breaker.
In bathrooms, new work requires 20A circuits and special rules as to what else can be on a circuit that feeds bathroom receptacles. However, these "new work" rules apply only to new construction and serious remodels; certainly not a fan replacement.
By the way, if you were to replace the switch loop with 12/2, you must use 12/3 instead, to comply with the new rules requiring neutral to be provisioned to switches. it would be better to run even more conductors to control fan and heater separately, but 12/4 is the largest size of NM cable reasonably available. You can't run 2 cables because current can't go down one cable and come back the other.