I guess a closer look at the ICAO guidelines could help here and might lead to a solution. Annex 14 states, what the prerequisites are. Now…as stated somewhere else, the game intends to be a tycoon game, not a simulation so we should not take these literally. From what I see the ICAO defines some danger levels depending on the type of plane and this goes from 1-10. Level 1 being planes up to 9m body length and things like an A380 are a 10. Basically it says that help must be given within 3 minutes and the danger level somewhat translates into how many people might be in need of rescue. The bigger the plane the more obviously. And this is translated into requirements in fire cars and their ability to extinguish fires.
In ACEO we have GA/small commercial, then medium and then large planes so having a level of I-III might be simulatoristic enough but not too complicated. The game just checks what stands you offer and then requires you to build the proper infrastructure by informing you via e-mail or denying opening the stand unless you have the proper number of fire cars at hand.
No research here as this seems to be one of the basic requirements of operating an airport at all. The tutorial should reflect this and urge you to at least have a shack with a fire car in it or have one mandatory before opening the airport. I can understand people getting angry, when the game whacks them with random events and they even had no chance in avoiding it like hitting them while they research it.
So solution: do not research fire engines and make them mandatory from the beginning of the game once you open the airport. Maybe you will get the first fire engine for free as it is a muncipal one - just have a parking lot ready so you obey the ICAO standards. The bigger the birds you want to land there, the better your infrastructure will be.