Please excuse posting to this old thread, but it was recently mentioned in another... (please split this off if you feel it should start a new thread.)

The Silky fin type and Shark body type can only be produced from eggs (unless you already have one in your tanks.) i.e. a Silky and Tiger will make another Silky, or a Shark and a Catfish will make another Shark, but you need one to start with.

I've also determined that starting with a twin-fin and silky, you can produce every other fin type (assuming the proper environment) within four generations.

Starting with a Spotanus and Shark it takes five generations to produce all of the body types.

I haven't yet determined the mininum number of generations to produce any specific species...