monsta!

Cassidy is in charge of the teevee tonight so currently watching Monsters Inc. (again) and it got me wondering, how does reproduction work in their world? 

Are the monsters all like dogs (same species but different genes), which allows them to interbreed?

That seems unlikely given the huge physiological differences seen between them. 

Look at the head of the company, the 8 legged spider monster Henry J. Waternoose III vs Mike.

This leads me onto my next point. 

How are stable population sizes maintained if there are only a few breeding pairs for each type of monster? 

What sort of evolutionary situations would have had to be seen for such a large variety of equally intelligent monsters to develop?

Or am I just being a very, very sad and lonely man?

Sully and Boo reunited in a little seen deleted sequence. Possibly