Would I be able to discard cards from my hand for the effect of Guzma & Hala when my opponent has Shiftry's "Shifty Substitution" Ability in effect?
No, all you can do is draw 3 cards.
Source: TPCi Rules Team (2020-12-10)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH4_12
SWSH4_12
SM12_193
SM12_193