You can, you always can. But this will be costly.
There is always a small increase in cost each time you add a fleet, but big penalties applies:
- from 2 to 3 fleets under 70 planes
- from 3 to 4 fleets
- from 6 to 7 fleets (iirc, not completely sure about that one).
Penalties 2 and 3 are getting proportionally bigger and bigger as your overall fleet is larger. Ok at 200 planes, boring at 300, dangerous at 400 and almost killing you when larger than that. The 3rd probably as well, but I would really recommend not to try it.