Is there a certian amount of medication that too much and is bad for fish?
Sure. Infact just like human meds I would be careful of what you mix as the resutls may not be desirable. One of the most direct effects on extra meds will not be on the fish directly but on the bactera that live in your substrate. Most of the meds that are used are antibacteral thus they kill not only the bad bacteras but they also can do a number on the good bactera that make up your bio filter.
This is one reason a quartineen tank would be of benifit so you can dose your meds in that tank with your fish in it vs medicating the main tank.