I'm working on a project where we need to include pictures of people, however, they need to look as if they were taken on a lower resolution cell phone, like an iPhone. How can I work with stock images in PS to accomplish this?
Simply changing the image resolution, even adding fake aberration, won't change the lens's focal length. Phones look like they do because they have really short lenses - the famous selfie 'big nose, little ears' look - because you have to get far too close to crop to only the face. These things are designed to do that at arm's length.
You cannot alter the perspective after the fact; you can only do that by moving the camera before taking the picture. You can correct to give some fake lens distortion, but your backgrounds will look to be at the wrong distance.
People do tend to get a bit upset if it's discovered phone photos weren't actually taken on a phone. There's been a lot of this recently - one story from the original photographer about a photo used by Samsung, pretending it was from a phone... Samsung used my DSLR photo to fake their phone’s “Portrait Mode” Google has many more examples.