Hi Sully, thank you for the kind words! I'm really glad Impulse Drive is scratching that itch for you. This is definitely a great place to discuss Impulse Drive and its rules and I'm glad you asked that question.
By default, players are assumed to only be able take Moves from their own playbook to protect fictional niches for other players characters. Moves from other classes or new custom archetype Moves is one of the first places folks start drifting - so much so, there's a Move in Drifting that addresses this exact idea!
Atypical Archetypes is just my spin on taking Moves from other playbooks though. Your group may play it differently! For instance - a fictional change for a character may result in them getting a Move from another playbook.
There's also a rule that each player should have a different playbook, but the folks over at the Merely Roleplayers podcast had a season where 2 players played Mystics at the same time and that was a lot of fun seeing the difference between them.