With Florges' "Flower Picking" Ability, can I use it when my opponent only has one card in their hand? Or can I use it to shuffle in only one card instead of two if my opponent has two or more cards?

If the opponent only has one card in their hand you do as much as you can, so they reveal & shuffle in the one card. But if your opponent has two or more cards in their hand you must choose two of them, not just one.

Relevant cards

SM12_152