There are other effects. First, the more planes you have, the more painful the multiple fleet penalty. At 198 planes, it must be hurting, but it's not an absolute killer yet. But if he grows at 600 planes, he's going to have problems.
And if the company is very-well led, it can work anyways. Fleet types is just a parameter. 4th fleet type is like 2000NM travels with 70-seaters : dont do that before being an expert. When you begin to get good, and when you know what you do, then you can try to bypass some of the advice given in the forum. Don't overdo it, though(that SW3 flying 2000NM with a tech stop is definitively not a good idea. I just keep it because it's fun. But I'm losing money there).