From the previous topic:

103.4d is an alternate mulligan rule, meaning it completely replaces other mulligan rules.

Rule 103.4a and Serum Powder:

103.4a If an effect allows a player to perform an action “any time could mulligan,” the
player may perform that action at a time he or she would declare whether or not he or she will
take a mulligan. This need not be in the first round of mulligans. Other players may have
already made their mulligan declarations by the time the player has the option to perform this
action. If the player performs the action, he or she then declares whether or not he or she will
take a mulligan.

Serum Powder
Any time you could mulligan and Serum Powder is in your hand, you may exile all the cards from your hand, then draw that many cards. (You can do this in addition to taking mulligans.)

Serum Powder works in Commander.

Daniel Kitachewsky
L3, Paris, France
Rules Netrep