I want newly produced ships to join an exsisting fleet .
Set the factory's rally point on the fleet (you can click the fleet icon in the empire tree as the rally point target). Note that it sends the ship to wherever the fleet is, but if you have more than one fleet in the same gravity well, there's no guarantee that the new ship will join the fleet it was rallied to (no idea why or how it's decided, just how it is).
Also when a ship joins a fleet will it automatically assume the same engagement rules?
It should in the beta patch. If it doesn't, bug! They didn't before the beta patch, though.