Currently Pokemon go works for IOS 9 and above. HPWU is built in collaboration with Niantic so HPWU and Pokemon go will probably have about the same compatibility. By the time HPWU is released other devices might have been discontinued or they might operate on a completly different system, but either way it’s highly unlikely that anything below IOS 9 will be compatible with HPWU.
Wizards unite on iOS / Apple compatibility
Zeroghan Selected answer as best