If I use Regidrago VSTAR's "Legacy Star" Ability, can I choose to discard less than 7 cards from my deck?
No, you must discard 7 cards if you are able to do so.
Source: TPCi Rules Team (2024-07-25)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH12_136
SWSH12_136