Using 2x CAT6 cable for 24 V hotend
Do not consolidate ground wires, use as many as the positive ones. In fact, use one wire of each twist for ground and one for the related positive, so that they balance each other.

Mixing voltages is fine, the CAT 6 cable will hold 24 V fine.

Concerning the number of wires, you should check the allowed current per wire and calculate the required number of wires.

Each pair can carry 0.36 A. 50 W is 2 A, you need 6 pairs for the heater.

You get two spare pairs, which can carry in total 0.7 A at the voltage you prefer. You could use two for the thermocouple and two for the fan, or two different fans.