Both the Slaadi and Slann originated from White Dwarf / Games Workshop in the UK sometime in the late 70's or early 80's. The writer Charles Stross who contributed the Slaad to White Dwarf's Fiend Factory claims it came to him in a fever dream.
I know even less about the Slann. I think they first appeared in a Warhammer Battle Suppliment in an encounter called The Magnificent Sven.
My question is basically in the title. Do these two very similar trans-dimensional races of frogmen have a common ancestor? Is there a common ancestor based in actual mythology?