During the Quick Look, Jeff had the Credit denominations popping up for each kill in the bottom left hand corner (like in Matchmaking). Was this because he had scoring turned on? 
 
I'm doing Heroic with no scoring and I'm not getting the little notifier. I get Credits when I leave the game, but does that include all my kills? Or just the Commendation/ Medals and then a basic Campaign number? Or do I need to turn on Scoring to get Kill Credits?

You're getting scored on everything you do always. Kills, completing levels, getting commendations, and all that. You just have the score in game and at the end of a level turned off by default when you're playing alone or your first time through. You can turn that on from the options when you go back through again. On my first playthrough on normal I got all the way to Sergeant without even realizing it.

The ribbons are Commendations. Challenges are what you see in the menu when you press Start, down near the bottom. 
 
You get scored in campaign with it on or not, getting credit bonuses for special and multikills. You earn the same medals as you would in MP in Campaign, too.

You do always get credits, but you can turn it on (first play through or any more) Just turn on the Meta scoring and the credits will also come up with the kill scores.