If I move Kato the Arm to a battlefield and then play an Ambush unit to that battlefield, can I give the Ambush unit the additional Might and keywords from Kato the Arm's move ability?

Ruling: No. You cannot target the Ambush unit with Kato the Arm's triggered ability because targets must be chosen when the ability is placed on the chain, before you have the opportunity to play the Ambush unit as a reaction.

Sequence:

• Move Kato to a battlefield, triggering his ability
• Choose targets for the ability (must choose "another friendly unit" that is already on the battlefield)
• The ability becomes a chain item
• Now you may play the Ambush unit as a reaction
• The Ambush unit enters play, then Kato's ability resolves affecting the previously chosen target

Nuances:

• If you have no other friendly units on the battlefield when Kato moves, the triggered ability is immediately removed from the chain per Rule 402.3, as there are not enough valid options to make legal choices.