Take a look at the original sentence again:
Ryan is highly allergic to cats, he requires
at least twenty god-bless-yous every time he visits Mariko and her seven
Siamese.
You chose to fix it the correct way:
Ryan is highly allergic to cats, so he requires
at least twenty god-bless-yous every time he visits Mariko and her seven
Siamese.
Ryan is highly allergic to cats is a main clause. He requires at least twenty god-bless-yous is the second main clause. You are always correct when you use a comma and a coordinating conjunction like so between the two main clauses that create the comma splice.