Some places will not take anything bigger than a fifty. It is not so much that they are afraid of counterfeits. Counterfeit bills are readily detectable by the pens used these days or just a close look by someone with 5 minutes of training.

The real reason is that some retailers (usually the smaller ones) keep a very minimal amount of change on hand as an anti-crime technique.

Personally, I think you would be better off with US twenties and hundreds as they are almost a universal currency, and/or local currency.

A tenth ounce gold coin is worth less than $150. I don't know what you think that is going to buy you in some foreign country in the middle of nowhere, assuming the tribesman involved even recognizes it as something valuable. I doubt desert tribesmen have much experience with modern US gold bullion coins.