You should feed it right away.  Pound some holes around the base of it and pour in some Espoma Flower-tone.  The tree spikes are too strong for your recently planted Jane.  Here’s a blog post all about spring feeding.  Pruning should be done right after the flowering is finished.  Magnolias grow their bud for next spring’s flowers over the rest of this season.  If you prune later, you’ll be cutting off the latent buds that will be next spring’s flowers.  If you prune when it has just finished flowering, the new growth will have a chance to produce buds.