OK, let's not get into the whether or how, but let's just assume something happens and these four airlines are involved in separate mergers, as above.
Taking the second first, as it's the easier of the two to work through. You can see that both DL and CO are working towards the same fleet strategy - 777, 764, 757/767 and 73NG. The fact that the 777s and 757s is not likely to cause huge trouble. The only major problem I can foresee is that both CO and DL have dominant positions at the two NY airports and that may have to change.
And now the big one. I have a vision of the head of AA's fleet planning division being brought into Don Carty's office, sat down and given a drink. And being told that his lovely fleet plans, carefully worked out over long hours, are in tatters and now he has to work on NW's fleet. And he bursts into tears.
But seriously, the main aim will be to get a fleet which comprises thirteen different types (pilot type ratings) down to about five. These five are likely to be the 744/744X (as NW already has a strong Pacific network, which will need 747s), 773X/772 (the 773X being an ideal replacement for NW's 747-200s), a 200-250 seater, the 757 (the only common type), the 737NG and the F100.
The F100 should stay as it is a fairly new aircraft, but there's no sense in selling them as the collapse of Fokker left them with a low resale value. Anyway, they would make a good replacement for some of NW's DC9s. The 727s and A320s would have to go, much as many people like the 320s. They may not go immediately and most efforts would go towards replacing the DC9s. However, there is no way the 737NG would give way.
However, and finally, Airbus could well find a place in the 250 seat category, the AB6s being returned and the A332 replacing them and the DC10s and 767-300s.