The only airlines that are Boeing or Airbus exclusive are low costs (like Ryanair or Spirit Airlines) and tiny airlines operating a handful of planes. Otherwise it's all mixed fleet (Delta, Air France, JAL, ANA, Lufthansa, UA, etc.). If you separate wide-body and narrow-body then your statement is more correct: as going narrow-body Airbus and wide-body Boeing (or the other way around) is not rare.
The threshold I've read on airliners.net is at about 30 frames. Below that costs of a sub-fleet are too high and you should aim for commonality. Above 30 frames you should pick the best plane for your routes, regardless of commonality.
So for example running two fleets of 30 A350-900/1000, and 30 787-9/10 would be perfectly rational. But having 50 A350-900/1000 and introducing a sub-fleet of 10 787-10 is unlikely to be a wise choice, even for routes where the 787-10 would beat the A350-900.