Welcome to the forum! I'm working on a GameCube portable right now, good stuff.
That step-down regulator looks like it should work. I'm not sure how the size compares to the custom regs most use here, but assuming size isn't an issue I don't see why they wouldn't work. Only pro I guess to making your own is they don't have to be adjusted/programmed to get the right voltage, they are hard wired to regulate the voltage to the desired amount, so it may be a bit more reliable because it's a master of one trade so to speak instead of a jack of trades (er, voltages...), but that's just speculation. Some of the other guys here can probably illuminate more on that, I haven't had to mess with custom regs yet.