Painting spindles, runners and risers, the actual part between each step and on the ends, is time consuming. The best way to deal with these items is to mask off the finished wood, first with tape then with paper, and carefully paint with a brush.
Use a blue safe release painters tape. Regular masking tape could damage the freshly applied floor finish.
A good paint will stick to stain without any problems, but excessive amounts of the floor finish (poly) will need priming. Typically the stained areas will require two coats for complete coverage.