Undervolting PC Fan from 12V to 7V
This is not a great solution. The 5V rail is not intended to sink current in most multi-output power supplies, and it may not be able to do that effectively. It may do weird things like cause the 5V line to float up to a higher voltage or even damage something.

A more solid solution would be to use a DC-DC converter to step the 12V down efficiently. You can get them cheap on Amazon.