Could it be a Krupa kit instead?

If i recall right the second floor tom on Krupa set was a 20" floor tom...Mikey

Over the years, Slingerland called many different configurations of drum sets a Krupa something or other. The sizes of these various sets might have been the same as those that Gene Krupa actually played. But, most of the drum sets were comprised of drums with dimensions that Krupa did not play. There were also "junior" Krupa drum sets made up of lower line drums than those played by the artist himself. Are you actually asking the dreaded "catalog-correct" question concerning the drums in a picture of Gene Krupa in a catalog? If so, you should look closely at those pictures, and read the text very carefully. Some of the descriptions spell out that some substitutions of different sizes of drums could be ordered.

Now, to answer your questions as asked......... Krupa often times used a 20" diameter floor tom as his second floor tom. I think that he sometimes used an 18" or even a 16" drum for the second floor tom. Will adding a second floor tom diminish the value of your "Krupa" drum set? Of course not. If you had to, you could sell the added tom by itself for at least what you paid for it. 16" older Slingy WMP floor toms are quite plentiful. 18" versions are not as plentiful. 20" floor toms are scarce. Even when you find a tom to add, it's a big crap shoot to find a close match of fade and patina to the drums you already have.