I am not saying that the system is perfect, or that it can't be changed / improved.
But it did does succeed in slowing down growth of airlines. Otherwise, the airlines would have 1000 aircraft in 10 fleet types in first 3 years of the game, and then, whet is there to for the remaining 30+ years?
As it is implemented, you have a tradeoff. You can either grow very fast with > 3 fleet types, you can even outpace others in overall profit, but you will eventually be sacrificing your profit margins as your airline becomes larger. Or you can go with no more that 3 fleet types, with slower growth, but you can maintain better profit margins, as your airline becomes larger.
I am just giving you advice how to play under current rules. You need to be always moving toward 2 fleet types. Your 3rd fleet type, that you are planning on replacing, needs to go down in numbers, preferably to zero, and then you can comfortably bring in its replacement, without having to do 20 hours of scheduling in a single day.
Looking at your airline in GW1, you need to pick which one you are going to stick with longer, either BAC One-Eleven or 722 (or you should have done that, a while ago). You should have been increasing the numbers of the one you are sticking with, decreasing the numbers of the one you are getting rid of, so that when your A320s start arriving, you can put them into service right away.
All this would apply, if your airline was 2x the size it is or larger. With 214 aircraft in operation, going to 4 fleet is not going to kill your airline. This is generally an issue for much larger airlines.