Are cosmetic hip implants safe? Is it a common practice? If not, would butt implants help give me more of an hourglass shape?

Indeed hip implants are a relatively safe and effective surgery. As well, buttock implants, especially round shaped, will yield some hip augmentation and thus a more hour glass figure. Most patients are satisfied with their new "figure" after just buttock implants but some indeed desire and benefit more from the addition of hip implants.  Glad to help. 

Hip implants almost never look natural.  Most surgeons, myself included, use fat transfers to the hips and that gives a beautiful result!  I love performing this surgery because it really makes for a dramatic change and most patients are extremely happy with their results.

Hip implants are an infrequently performed procedure as injectable fat grafting today as eliminated their need for many patients. They are only used today for very thin patients that have little fat to transfer. Unlike all other body implants they are not placed under the fascia but in the subcutaneous plane and their location exposes them to more motion and flexion that most other body implants. That does not make them unsafe just that they can have a higher complication rate.