Is horseback riding a bad idea if you have scoliosis?

No. You are only limited by discomfort. Riding itself won't change or effect anything in most cases.
Not necessarily. If you are skeletally mature with a mild curve, the curve is unlikely to progress as a result of horseback riding. You are at a statistically greater risk of back pain then the general public though and as such should observe good ergonomics, and practice core stability.