Safe, safe, not that safe at night... it is a bit rough anyway don't worry.
The metro is safe but terribly crowded, the northern line 13 is divided in two branchs and "Porte de Saint Ouen" is in Saint Denis branch (dark blue in the electronic panel).
So avoid the rush hours in the northern line 13 (stations after Saint Lazare).
It is already very busy in off peak hours, so imagine how bad are the rush hours.
To go in the Louvre change at "Champs Elysees Clemenceau" for the line 1. The metro station of the Louvre museum is "Palais Royal Musee du Louvre"