@1bit, @Rno, the 6.5V rating is an ABSOLUTE MAX rating and should not be used as an operating value. The recommended input voltage is 3.6v to 5.5v. Furthermore, the challenge with using a fuse is that it is a current protection device requiring a tripping delay which is typically in the order of seconds. The Photon may be damaged before the fuse trips. The most common failure mode I have seen with the Photon regulator is self-destructive feedback caused by poor USB power supplies or long wires causing inductive “ringing”. This was addressed in another topic.
Do consider your power requirements as a whole, including Photon and all peripheral devices. Your “whole system” requirements should dictate the power supply design you need.