How far ahead can I make the gnocchi? Also, will it get tough if I overmix?

Heidi Swanson at 101Cookbooks has a good post that walks you through making gnocchi: http://www.101cookbooks.com/archives/how-to-make-gnocchi-like-an-italian-grandmother-recipe.html (It's not a sweet potato version, but you still might find a helpful tip or two.)

Thank you--so--any cook's tips for what is the max amount of time to mix the sweet potato with flour and egg and not have it get tough? Or any cook's tips about what to look for in consistency so that it stays tender?

yes it will get tough... You can make gnocchi ahead and freeze it and take it out wihen you want to cook it.