Before anything else you need to take a look at the 24 volts feeding your LM317, While the +12 volt rail likely has plenty of current note the current available from your -12 volt rail. I would venture there won't be much. Generally less than an amp. Also the -12 becomes the common for that independent supply and can't at any time share the supply common. Your PCB GND and 0V. Fusing would depend on what the -12 volt rail is capable of for the supply, I would venture a guess that the max would be maybe .5 amp.
Yes, in the event of overload, if the PSU is built to ATX design specifications (and new enough) it should shut down with any excessive load.
I would fuse below the rated outputs. The problem with how many ATX PSUs are rated is the currents listed on the case for various voltages don't always consider cross loading. I would fuse maybe 5 amps below maximum ratings. Consider the wires from PSU to jacks are likely AWG 18 so make sure the parallel lines can handle what you fuse for also.
Just My Take
Ron