Fitter suggested too short a stem

I had my bike professionally fitted a few months back and the guy made some changes to it. One of them was going from a 100mm stem to a 60mm stem. The change felt better for my back, but the bike feels really hard to control and I can barely ride it while standing. I tried going back to the 100mm and just pushing the saddle forwards to compensate and have been feeling the position to be about as comfortable, but the bike handles well again. The question is: is pushing the saddle forward by more than suggested to me by the fitter (almost all the way) a bad idea? will this have a negative impact?