Originally Posted by zube

Impeditum is a dwarf rhododendron, not satsuki. Satsuki are R. Indica, I believe. Your plant looks good for bonsai though, it has a good single trunk, which aren't common in 'Impeditum'. You can prune it as hard as you want. New shoots come in groups of five, and should be reduced to two (usually one to the right and one to the left). It will have bluish-purple flowers in the spring. Pluck off all of them after they wilt.
Good luck,
zube