I know what you mean about the hip flexors.

My left side was so-o-tight a year ago but, Pilates has really helped me. Core strength is the main goal of Pilates - my hips are finally even! I still have problems with the hip flexors "firing" when I get muscle fatigue in some of the exercises. Tightness and spasticity can be helped by stretching and strengthening the muscle.
I would highly recommend the Pilates routine. I go to a private studio which is a bit more expensive than a community center. I like the smaller classes (5-6 people in comparison to 20 at the community center). The studio director is aware of the MS and helps me to accomodate some of the weakness.
Sharon