I believe you might have a problem with your FD because it is meant for a normal crank. The curvature of your FD might not be compatible with the smaller diameter chainring. Usually when we change out cranks to compacts we recommend changing the FD to a compact compatible FD. If not we gently bend the FD down slightly just to better match the profile of a smaller chainring. Then it's a matter of adjusting with the set screws.
As for chain length it's something that you have to judge because every set up is different.