Hi Wonton. Welcome.
Yes, the CT (combat theater) has a limited planeset that is meant to be historically accurate. In the MA (main arena) all planes are available all the time, except for perk planes, which you have to save up points to fly.
Evening up numbers in any arena is generally a good idea, but in the MA it usually doesn't matter too much if one person switches sides. Look for good fights in your country, and if there are none, switch teams. Or eventually when you find a squad or just a few guys you work well with, stick with that country for a while.
And as for shooting a smoking plane, that's often okay. Planes can take engine damage, start smoking, and still be dangerous for a few minutes before their engine dies. Obviously, if the plane is fluttering into the ground don't shoot, but if its still flying and got all of its pieces, shoot away.
