I am afraid there is a difference...
Extreme: farthest from ordinary
Diversity: displaying multiple points of difference
These are both qualitative terms to an extent; however, the former speaks of only one area of difference, whereas the latter is an umbrella term for all points of difference in a population, hence its tie to the term polymorphism: many forms. Your argument seems to say that the individual keeper has his or her own definitions of morphs and genetics, this is of course plausible; however, we must use the most logical and agreed upon terms when discussing such areas with others. How many different names do you think me and my lab partners have for all the machinery and tests we perform? Tons, but the "official" name goes on the research paper.