Scoliosis does NOT prevent you from meditating! Meditation is less about sitting cross legged on the floor and more about focusing on your breathing and clearing your mind. I often do my medutating laying down on my back or side, and I have also done it sitting in a chair, standing, running on a treadmill; all you need is a comfortable position that you can stay in for the duration of your meditation session.
No, although it might be more painful to sit in the classic position, you can really sorry however you want to. Besides, despite what popular culture often espouses, mindfulness meditation is not about achieving any sort of state of calm, clarity, openness, etc.. It's about being fully aware at whatever you are feeling. So if your back hurts, you can still meditate on the feeling of pain itself. I highly recommend listening to some Mingyur Rinpoche on the Palpung Lungtok YouTube channel. It should make meditation much easier to understand for someone getting started. Hope this helps!