You can prune mimosa safely, specially in fall. When temperatures raise and light hours increase, she'll be back.
Some time ago, and after losing a one year old mimosa tree to winter cold, I brought my next mimosa indoors in mid november (northern hemisphere). Pruned generously branches, and although it might not be necessary if the plant is healthy and close to one year of age at least, I left a few leaves. First surprise: just following the raise of temperature as she arrived indoors, in a couple weeks she was full of new leaf shoots. They grew a little elongated and pale due to not receiving enough light, and eventually started falling a little, but by march I moved her outdoors again and soon received lots of beautiful new growth.
So feel free to prune mimosa at your leisure; taming her into a shape or another, though, might take time and patience.
Mandrake