Eating flames is unfanthomable to begin with in real life. They argued that dragon slayers were taught to be able to stomach dragon flames and thus could eat anything they want. God flame was stronger so in order to eat that you had to rebuild your container, which must be emptied or else the residual dragon flame would burn your innards.
By extention, this would apply to any other "incompatible elements" until the user is accustomed to it. The question would come down to whether any two elements would be mutually incompatible, and the time/training required to actually pull it off.
I don't think it was stupid at all. In fact, it's one of the few things in FT powerups that have a decent explanation.
I agree that the use of the word "element" in my previous post was factually erroneous.