So if the main arkham board is already at monster limit, and a flying monster on an expansion board moves (to the sky), what happens?

Does

a) the monster move to the outskirts instead of the sky, or

b) does it move to the sky (which would be illogical as the main board is already at limit)?

Thanks guys.

According to the rules, it would fly to the sky. If the board is at the monster limit then it would go into the outskirts.

There is, however, nothing to stop you creating a house rule that each expansion board has it's own sky.

a) happens. The Sky is technically located on the Arkham board, and there is a limit there. This also applies to "move directly to" monsters that would have moved somewhere in Arkham.