From: Cottesmore, Rutland
ORIGINAL: Capt Hornblower
As to your 2nd question, the answer is a qualified "yes". If you disband 3 of your squadrons and your HQ unit into one of the group's squadrons and tell the disbanding units not to re-form, then you're essentially permanently combining the group into one squadron. If, however, you let the disbanding units re-form in 120 days, then you can reconstitute the group with replacement aircraft. Since the US eventually gears up production to a point where there's a surfeit of aircraft, there's no reason to permanently remove squadrons that could prove useful.
Not sure were you got that from, the answers is no you cannot make one big group, which I think is the question. As you said you can disband (or withdraw) a unit into another unit that has the same a/c but you cannot exceed the units normal compliment.
At game start you do have lots of air units in the US with few a/c, some cannot leave the US, some are HQ restricted. Initially for those that are going to be withdrawn in the first 4-6 months, all permanently restricted and all HQ units set replacements off and give them max pilots set them to train. They train pretty quickly even without a full compliment. These units are not worth the effort to ship. Once you start to build up a pool you can then decide what you are going to do with the HQ restricted units. The HQ units are permanently withdrawn in June.