this game isn't very smg friendly IMO.
I beg to differ, using the smg's at their appropriate range you will outgun your opponents 9 times out of 10. just last night using the mtar in FFA I had 3 straight 30-1 games running and gunning with specialist killstreak.
Now to try and answer your question, If your coming from hardcore you will be use to people dying much faster of course, I recommend slapping rapid fire and muzzle break on your mtar for starters. My preferred setup is 3 attachments on the mtar, Grip, Rapid Fire, Muzzle Break w/ focus perk. Also consider running scavenger or running specialist with scavenger in the first spot.
The best game mode for rescue kills I have found was domination. What I did was follow groups of teammates who were running to flags (these guys tend to have blinders on sometimes) I basically ran with them while I scanned all the nooks and crannies that people like to sit or pop out and defend from. Having the insanely fast kill rate of the mtar with rapid fire allowed me to drop many an enemy well after they started shooting at us.
If following randoms around isn't your thing, I also found that on many games B flag was where all the action was. Go to where you hear all the gun battles and mow down the enemy. I seemed to get more rescue kills when I wasn't consciously thinking about them but instead just going to hot spots and killing enemy.
Hope this works for you, best of luck.
EDIT: You also might want to try Heavy Duty game mode solely for the reason of the increased health, giving you a litter bit more chance to rescue a teammate.
I agree that Ghosts is not as friendly to SMGs in that you can no longer hip-fire your way from one end of the map to the other. LOL! BO2 made a mockery out of everything combat related. Ghosts has done a much better job of balancing the weapons. I do however have a slight problem with Ghosts snipers though. I only say that because I'm wrecking shop with them in this game and I've NEVER sniped before. I've only been playing FPS games since BO2 and there's no legitimate reason why I should be having so much success QSing outside of it just being TOO EASY for some inexplicable reason.