Am I allowed to do other MLMs, or do I need to be exclusive to Nu Skin?

Yes you are allowed, but with some caveats. As stated in the chapter 5 section 2.2 of the company's Policies and Procedures:

As a Brand Affiliate with a Ruby Partner (Executive Brand Partner) or higher pin level, you are reasonably expected to exclusively sell Company Products, train Brand Affiliates on your Team and promote the Company’s business. Therefore, as a condition to receiving ongoing breakaway compensation on Brand Representative Levels 3 through 6 on your Team, and recognition as a Ruby Partner (Executive Brand Partner) or higher Brand Affiliate leader at Company events, you may not be engaged in any Business Development Activity for any other Direct Sales Company.

So yes, you can do other MLMs. However, when you reach the PIN title of Ruby Partner, you are expected to exclusively sell Nu Skin products. Go ahead and check out the Policies and Procedures for more information.