Can I not so fast? A mind splitter? He is targeting an Adaptatron that's in my hand.

No, you cannot use Not So Fast to counter Mindsplitter's ability in this situation.

There are two primary reasons why this interaction is not legal:

1. "Friendly unit or gear" Requirement: According to its card text, Not So Fast can only counter a spell or ability that chooses a "friendly unit or gear." While Adaptatron is a unit card, Mindsplitter is targeting a card in your hand, not a unit on the battlefield. Because Adaptatron is not a unit on the battlefield, it does not qualify as a "friendly unit" for the purposes of the Not So Fast targeting restriction.
2. Targeting Restriction: Even if Adaptatron were on the battlefield, Mindsplitter's ability chooses a "card" from the hand. Since Not So Fast specifically requires targeting a unit or gear, and cards in your hand are neither units nor gear (they are just cards), the ability does not meet the necessary criteria for Not So Fast to target it.

This is my interpretation based on the rules governing targeting restrictions and card types. No official FAQ exists for this specific interaction, but it follows the established rules that protection and targeting based on "unit" or "gear" statuses only apply to those objects while they are on the battlefield.