The Cooker or Feathers McGraw will get a spin-off.
Shirley is Shaun from A Close Shave.
Look here, Shaun is an eating machine of, err...MASS EATING! in A Close Shave, but in this series, "Shaun" is not that hungry, while "Shirley" EATS AS MUCH AS SHAUN in A Close Shave.
Therefore my theory is "Shirley is Shaun". "Shaun" is just Shirley's son, and Shirley doesn't have to be a girl - It says so on the Aardman website. Shaun changed his name and gender so Wallace and Gromit wouldn't find him.
All the sheep except for Shaun and Timmy are female.
Because that's how a farmer would do it, to prevent fights between males. Yes, that means Shaun is Timmy's father.- Or, you know, both Shaun's and Timmy's mothers were artificially inseminated. Common practice at farms.
Why Shaun is so smart.
He stayed a bit more in Wallace and Grommit's home, learning a few things from Wallace and Grommit. Mostly from Grommit obviously. Getting their tinkering capabilities and such, while slowly dropping his tendencies to eat anything. Soon after that he was sent to live with the rest of his flock Oop North. Some of the sheep were turned into dog meat.
In later media there are only eight sheep when in the first few seasons there are more sheep. It is possible that A Close Shave took place during those early seasons. Timmy's Mother isn't really his mother.
In one episode, her curlers come off showing that they are fake. The farmer is really Postman Pat.
