Does "not so fast" counter "Cull the Weak"?

No, Not So Fast cannot counter Cull the Weak.

According to the card text and official rulings, Cull the Weak does not "choose" or "target" a unit. The instruction to kill a unit is performed by each player during the resolution of the spell, rather than being a targeted choice made when the spell is played.

Because Not So Fast specifically requires the spell or ability to "choose a friendly unit or gear" in order to be played, it cannot be used against Cull the Weak.