Doesn't add up. Zaeed will only be Loyal to you if you let him be a huge douche during his mission and act like a mainiac.
I thought it was strange that Zaeed wasn't a good choice at first, too, but then I realized that when he tried to run a fireteam they shot him in the face and became one of the three most vile pirate groups in the galaxy. His resume's not very good.
Garrus on the other hand only lost his party when he wasn't there. It only makes sense that if you don't send him, your squaddies will die.
Not sure what the deal with Miranda or Jacob is though, since certain characters hate Miranda, and Jacob has no qualifications. He had his own game once, though, so there's that.
Grunt is a bad fireteam leader because he's less than a month old. And he's a Krogan. His dying is a bit weird on the second fireteam (shouldn't he shrug that shit off?), but the death from the first fireteam seems legit.
Both Zaeed and Grunt have good "Hold the Line" scores, though. Like, if you left them there all alone, they would survive no problem. Just by having them in the party, you prevent a total party wipe (in fact, I don't think it's possible for Grunt to die if you leave him behind). That means you can also set it up so that Grunt is the only survivor left, and he turns out to be too wimpy to pull you into the Normandy. Classy.
Also possible to leave Zaeed as the last guy, so he can go "Yeah, we all went into the Collector Base. Shot a whole bunch of aliens down there. And I was the only one who made it out alive. Team leader tried to get on the ship and I just pretended I was pinned by assault fire and couldn't help him, the sucker."