Is it safe to mix two different butt enhancers? Like can I pick one cream and a different brand of pills?

It is very difficult to determine the exact size and shape implant you will require to best match your ideal butt image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your buttock will determine, in many cases, the maximal volume per implant profile that you can accommodate. Liposuction of your lower back and fat transfer is also helpful as an adjunct to implants. Using “want to be” photos however are useful if simply provided to the surgeon.

Thank you for your question.
I would caution you to take the benefits of creams/pills to enhance your derrierre with a grain of salt, as upon close inspection you may find that the claims put forth are more than likely completely unfounded, with no sound research to back their effectiveness, and simply marketing gimmicks. 

It is a pleasure to answer your question. There are no magical creams or treatments that would help you enhance your buttocks. In my experience I would recommend you to either get Brazilian Butt Lift (BBL) or Buttocks Implants.
BBL uses your own fat to naturally augment and shape the buttocks. A board certified plastic surgeon first performs liposuction to remove fat from other areas of the body (abdomen, hips, back, flanks and thighs), then transfers that fat to the buttocks giving them a nice and natural look. Buttock Implant Surgery includes artificial Implants that are placed through an incision made either where the cheek meets the back of the thigh or in the buttock crease (where scars are not as noticeable). The surgeon then creates a pocket large enough to place the butt implants. Butt implants can be placed above or below the muscle. Please visit a board certified plastic surgeon for an assessment. Hope this helps. Good luck.