Just use 1 type of A/C for as long as you can, or a/c within the same fleet i.e A319,A320,A321..or Boieng 737 -3/4/5..md80,81,82, etc, etc. I had 1 bombardier 120crj in my fleet and alone it costs over 300 000 a month to have that in my fleet. It's something that fools a lot of people, myself included obviously, lesson was learned by almost going bust.
It's not easily apparant how costly fleet (un)communality is until your money just starts rapdily disappearing. There's plenty of threads on the forums about it. Fuel expenditure has only 1 solution, grounding all your flights, unless you can swap your entire fleet for modenr planes, age doesn't seem to matter...It's a cost that will always be there as long as your planes are in the air. But fleet communality expenditure is an area which you can directly control and easily, by using as few different types of A/C as possible. The larger a/c has a substantial cost of up to and beyong the million mark (not per a/c, and the incremental growth in this is small once you have started flying a type of a/c) for fleet maintenance/training, etc..so until you have a fleet of planes that can support that base cost, don't look to add another type of a/c with a similar monthly/weekly expenditure. Why spend 2 million when you can only spend 1?