I'm probably behind the times but I haven't heard it said that any amount may be harmful. Even fructose's harshest critic, Dr. Lustig, says that fructose's harm comes when an large acute volume of it overloads the ideal metabolic pathway and results in the using metabolic pathways with harmful byproducts. This is why he says whole fruit consumption is generally ok. The fiber in the fruit slows down absorption and gives your liver time to process the fructose without resorting to the harmful fallback methods.

There may be a threshold response, but from what I remember, the threshold might be low. I wonder if much of the fiber in tomato is destroyed when reducing it to a sauce?

Cooking doesn't destroy fiber, it just softens it. As opposed to pulp-free fruit juice, from which all fiber is removed, and is unhealthy. Even if tomato sauce were the equivalent of fruit juice, you'd usually have it with pasta, bread, etc. which all have fiber as well.