Hoover Dam generates gigawatts of power for use by its customers using Francis turbines. However, the power to operate the dam itself (a couple of megawatts) is generated by a Pelton wheel.
Given that both of these turbines are operating on approximately the same amount of hydraulic head, why the different types of turbine?
Based on these two quotes from your cites, I’d think the control power capability at a lower head pressure is one reason. They want power available as long as possible if the dam ever empties and earlier on in the process as it was filling.
I believe this was a problem of self building.
The smaller pelton wheels could be installed with construction equipment while working efficiently enough for their given parameters. Note that they power the 300 ton cranes used to install the main turbines, which I’m sure were selected for their calculated efficiency in the given parameters.
Apart from the Hoover dam, which I don’t know anything about other than the name*, P elton wheels (high pressure/low flow turbines) wind up being used in places where you want to just put something cheap somewhere easy. Francis turbines (low pressure / high flow) are pressure vessels, and need more care in construction and placement.
*Hoover worked in Aus before becoming President 
The big thing is that different turbine designs work better at different impedances, that is, different pressure to flow ratios. I think Melbourne alludes to this. People familiar with fans/blowers and pumps will recognize some of the patterns.
There is an exhibit of this in the courtyard of the amazing Deutsches Museum in Munich.
https://www.deutsches-museum.de/en/exhibitions/museum-island/inner-courtyard/exhibits/