I believe the yellow pump is limited by electrical load. The pumps take a ridiculous amount of power, 16.2 kVA max each. Between Y and B, you've eaten up 40% of your last engine's electrical power. In the worst case, you're trying to do a APU engine start with a dual fail on a hot day at FL 200, you only get 41 kVA, plus 5 from the emer gen.
My info says the EDPs can do around 45 GPM and the electric pump does about 8, depending on version. They're sold as Vickers.
The other thing to be aware of is that all of the pumps dump heat into the hyd fluid. The fluid even runs thru the electric motor. If you run it under low pressure, you you'll cause cavitation and heat damage to the pump. If you run it dry, you'll ruin the whole thing and possibly pull an AF.
I bet the reasoning behind the Y EDP failure checklist is that if you lose your Y EDP and the PTU can't bring pressure back, there's no need to risk breaking a third pump until things get desperate.