U and AA WILL merge and you're right, how many seats and who gets them when the music stops is a major question. On the mainline side, at least there's hope due to retirements. The combined entity will have about 1800 retirements in the next 5 years or so, due to age 65, but certainly more including early-out's and medicals. I'd guess 2200 or so. That MAY negate the requirement for less pilots and furloughing due to more efficient scheduling, but leaves little room for growth and thus hiring.

The regional side is different, in that age 65 attrition from the top is negligable and there are 6000 pilots and 600 regional aircraft between the AA and U networks as it stands now. At least half of those are 50-seat RJ's, many of which will be replaced with larger RJ's, but consolidation and streamling will likely require a smaller feed system. Some markets won't support 76-seat jets (or even turboprops) with minimum required frequency and thus will be abandoned.

Not sure how many aircraft (and pilots) are in the combined UAL/CAL and DAL/NWA networks, but I don't think it's 600. Sounds like those operations are already into their streamlining and reduction process though.