Yes, as long as the fuse or circuit breaker is rated for the smallest wire in the circuit, which in this case appears to be the 14 guage.
Suppose the 14 guage is rated for 15 amps and the the 12...
It really depends on what device you are connecting power to. It is not much of a concern if running the heavier 8ga. wire, into the lighter 10ga. wire, when dealing with a 12 volt system. There are...