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".