Can you still join the air force with minor scoliosis?

Yes. This may depend on the actual degree of your curve but as a general rule minor scoliosis should not restrict your activities or occupation.
Depends on degree. The following is cause for rejection: "there is lumbar scoliosis greater than 20 degrees, thoracic scoliosis greater than 30 degrees, and kyphosis or lordosis greater than 55 degrees when measured by the cobb method".
Yes. There should not be any issue with a small curvature related to idiopathic scoliosis.