Your grandfather sounds like a smart guy
You're right about the 500mA USB limit.
Your limit for the on-board supplies is 15W when driving with the External power jack. This is the limit of the 5V regulator (U7), so you've got about 3A at 5V. This has to cover the HiFive1 components as well of course, whatever remains is good for your shields at 5V.
Also, U8, the 3.3V regulator, can only supply 1A. So if you try to supply 3.3V shields with that supply directly, you have that limit as well.
EDIT: I was wrong. Don't do this! 5V supply is only as an output of HiFive1.
There is of also the external 5V pin on the header, so if you have a regulated 5V DC supply hooked up to your giant LED display (for example), you could use that as the power supply for the HiFive1.