Another idea, if we moved from limits being placed on when player orders aircraft (anytime, adding them to a queue) to limiting delivery rate, the limits on the delivery rate might be similar to how marketing works.
Number of simultaneous marketing campaigns is limited by the staff. As the staff grows, more campaigns can be started.
The similar system could work with aircraft deliveries. Maybe the brand new airline can only have 1-2 in flight, over time, it can grow to current rate of max of 6 aircraft being in process of being delivered from the queue. And for Mega Airlines, with huge staffs, it could go way beyond 6.