I'm fairly new to the game and therefore I would like to ask you, if the comparison in costs in the picture in the first post is right after you received your first plane of the 4th fleet, or is it with some active planes of the 4th fleet?
I did not know that the costs stack up this fast if you are operating various fleets. I wish I could have a look at these stats in the (now-closed) BW, where I operated 5 different A/C types with round about 350 A/C in total. Looking back at this I wonder if I could save a lot of money by flying some routes with A/C that are actually 30% too large for the demand on these routes but operating only 4 fleets.
As I said I'm new to this game and as of now I find it pretty hard to calculate. But let's say you're operating an airline with 500 A/C of 3 fleet types (737, 757, 767) and you are looking at further expansion in the short-haul/regional sector. The perfect A/C for this (regarding demand and frequency of flights to your planned short-haul-destinations) would be the ATR 72-500. How do you know in advance if buying 30 ATR's and therefore open up a 4th fleet will ruin your income or will generate more profit in the end? Is it just trial and error or can you calculate this anyhow?
In your situation, the better strategy would be to offload the 757 fleet onto a blend of the 737 and 767, depending on flight length, and then pick up the ATRs on the short haul.
In short, the penalty goes up exponentially in coorelation to your fleet size. At ~850 planes, I experienced about a 12-13x increase in total commonality. At 300 planes, you might be looking at 3-4x. One way to test it, if you happen to have one plane of a new type, is to add a route to the schedule and see how it changes, and unschedule it afer that.
In general, a 4th (5th and 6th) fleet type _can_ be made to work for fleet sizes under 300 planes. Once you get over 300, it will be very difficult to stay profitable as you continue to grow.