If the other geckos aren't showing any signs it might not be parasites. Her tail thinning may be from perhaps the other geckos not letting her eat(I'm not sure if a month or two is long enough for them to starve, but it could have weakened her significantly and she became stressed) or if she was old that could be part of it. Personally, I would get a necropsy done and/or get stool samples from the other geckos just to be safe. As for the affects on the balls, if it is in fact parasites it could very well affect the balls, the specific effect though depends on the type. So, as I said before I'd test to be safe.