1. Add your friend to your friend's list and go to the same sector or starbase and same instance (there is a way to select which instance you want to be in). Once you've formed a party, if you start a mission you both didn't complete, you should both get credit for it assuming you did any prerequisite missions. If you only play together, this should be perfect since you'll always be at the same point in the mission story lines. Where ever you two go, being in a team means the game will make you enter the same instance so you can do missions together. Experience is shared and you can set in the Social options what method you want to distribute loot.
2. User missions do scale I believe, and they are pretty damn amazing. Personally, I'm going through the official content before I touch user missions. The official content also starts off a little shaky but improves as you progress. I figure that user missions at the end would be the best because if you do them early, then the early official missions will be more disappointing in comparison.