So far I was thinking “IATA Aircraft Types”, as that’s what typically is listed on a real-world schedule. But even with that there could always be different capacities due to cabin layouts, so in theory, a player could always dump any given type into a pool and hope for the best 
That said, there will have to be some way to specify the intended capacity of a flight on a schedule without having to force the player to assign an explicit aircraft (which would go against the whole idea of auto-ops). Then auto-ops would have to try to minimise over- and under-capacities of already bookable flights once the flight takes off. Example: Flights are scheduled to operate with (the capacity of) an A321 but when the flight is supposed to take off, only an A320 is available, meaning some pax will have to stay behind. I need to be careful not to overpromise on anything here, because this stuff turns into hairy optimisation problems rather quickly 
In theory, a booking class feature could allow to make specifying capacities 100% flexible (just set whatever capacities you want on a per-flight segment-level), but that might be more flexibility (and work) than players can and want to handle.