For all of these, finding the right model/actor is one of the most important keys to a successful costume. That's the chief reason Holly Conrad's Grunt costume is such a success. The model/actor is tall enough and bulky enough that he can sell the character properly, and he really gives Grunt the presence the character needs.
I think a Garrus costume like the one pictured in the OP could work, but the model has to be really tall and thin, and possibly even female, to pull it off. Getting the waist and hips right is key, since there is so much bulk in the costume that hides the shoulder region. Thin legs are a must as well.
A good Tali model would be average height, but with a long neck and narrow waist. The hips can be padded. Thin legs and arms are a must.
The ideal Thane model is slender, athletic, and average height, and slightly long-waisted.
The perfect model for Samara or Aria has to have the right face shape, the right bust size, and a narrow waist. The costumes can camouflage most other areas, but having a nice hourglass figure is paramount.
I would love to see a good Mordin Solus costume, but once again I think the model/actor should probably be a thin, petite female.
Oddly enough, the most difficult characters to bring to life would have to be the humans. You really have to look like Miranda, Jacob, et al., to sell these characters properly.