Glad you're liking it
Ya, squad control isn't the greatest but I enjoyed being Commander Shepard "the greatest one man army". Of course, normal difficulty is a lot different from hardcore haha.
Shield guys can be tough and there are several approaches you can take. Tossing a grenade (James has them) at them usually makes them raise their shield in recoil and you can nail them then. Flanking them is also a huge bonus as bioware did a great job with their enemy AI. The enemy AI doesn't always know where you are (more realistic) so if they are distracted by your squadmates, flank around to the side and pop several shots into their side. You can usually get them stunned or disoriented that way and finish them off. The third option is to use sniper rifle and aim at their exposed feet. Usually a shot to the foot will make the shield carrier reel back allowing another quick finishing shot to the torso or head.
Make good use of the different kinds of ammo. I played as a soldier so I had flame, disruptor, and armor piercing ammo which I used to their maximum potential. If you have a squadmate that has a unique ammo type, be sure to upgrade it to allow squadmates access to it as well. It comes in handy in all situations throughout the game. Pretty self explanatory but here are some tips regarding the different types of ammo:
--flame is great for really anything. I used that the majority of the game regardless of the situation and I was just steam rolling through guys. If you get it maxed out then you can really burn through guys as they get constant burn damage as well as being stunned.
--disruptor ammo is great for taking down shields and will stun the enemy once his shields are gone. Not a great multipurpose ammo type like flame but excellent at quickly depleting a tougher enemies shields.
--armor piercing ammo, I doubt you'll guess what this is for
Great against armored units but nothing else.
Basically, I always had the flaming ammo equiped and switched to armor piercing and disruptor as the situation demanded.
As for the DLC. Not sure how far along in the story you are but you'll get a priority mission pretty early on telling you to go to Eden Prime. That's where you get the new squad member, assuming you bought and installed the DLC
You mentioned to me that you had already met Garrus so I'm assuming that you've taken down the brute. For those guys I found that a sniper rifle with armor piercing rounds worked the greatest. I was able to lay waste to them in 2-3 shots, and as you get farther along they come in packs of 2-5 so it's important to take them down as quickly as possible.
I played some more multiplayer today and actually started to really enjoy it. There is very little variance among the maps and objectives but it was fun to rank up the different classes and gamble with buying random supplies
I've played through the ending four times now lol (don't worry, no spoilers) and you can only get the two more common endings unless you import a character from ME2. I found that out through trial and error as well as a little help from google
Regardless, the ending still sucked but it seems that there is a prevalent theory floating around that makes sense but has some holes in it. I'm just really looking forward to bioware confirming or denying these rumors or by releasing the rumored "real" ending in a DLC.