I would probably say that it's alright as long as you're eating with your mouth closed and your hands are free from the food you're eating it with.
For instance, if you're eating with a fork and knife... be sure to put both down properly before signing. I've seen people sign without putting their forks/knives down properly. They end up hitting the forks/knives and it goes flinging in the air.
As for nail polish, I guess it depends on what kind of nail polish.
If it was normal nail polish (red, blue, black, etc), it would be fine. But if it was to flashy (shiny bright red, neon orange/pink, bright yellow, etc), it would be distracting. It's like clothes that interpreters wear, which have to be mild in color.
Of course, that would be good advice for an interpreter, but for a person who's not an interpreter... I see nothing wrong with how you want your nails. You're not trying to be a certified interpreter, but a normal human being who knows how to sign and want to talk with people.