I have 12-2 running to a 15 amp breaker. That constant runs to a box and has 14-2 running out if it to 6 can lights and a fan. It also has 12-2 running out to another light and outlet. Will this work, or do I need to change all the 14-2 to 12-2?

You can have mixed wire sizes in a circuit. Since the smallest wire is #14, the max breaker size will be a 15 amp.

The downside t having #12 at the breaker is someone in the future may assume the entire circuit is #12 and change the breaker to a 20 amp.

You should clearly label the #12 at the panel that the circuit max is 15 amp.

A better job would be to have all one wire size thru-out the circuit.

If you change it all to #12, then you can have a 20 amp circuit.