To simply answer your question, yes you can only have the hump reduced but you must understand that there should be harmony between the bridge of the nose and the tip. The nose is the most central portion a persons' face and if the nose is not harmonious among its parts it can disrupt the overall facial aesthetics. Rhinoplasty is not simplistic.
1. When the hump is reduced this creates a wide nasal bridge. Osteotomies help narrow the nose.
2. Once the bridge is narrowed this may create a mismatch between the bridge and the tip. (the tip may look bigger). So some refinement of the tip may be warranted.
This is cosmetic procedure where you have input but during your evaluation your surgeon should direct to the pros and cons of "selective" alteration of one portion of the nose.
Rhinoplasty is an elegant operation and when planned and performed correctly will produce excellent results.