10kclockman, I'd say that the higher your resolution, meaning, the more megapixels you have, the more detail you will get in your final model. I would lean toward a professional camera, were I you. I've never used an iPhone for this sort of thing but I wouldn't call it a pro camera. It's nice, but it's not pro. You want a camera that you can control in terms of shutter speed, ISO and aperture, not to mention white balance. And you want to shoot RAW and then convert to TIFF before you go into Photoscan. That way, you get the very highest quality that is available.
Now, you don't HAVE to have any of this. Clearly, you can use an iPhone and get a decent result. You've proven that. But you asked about improving quality so that's my 2 pence.
Furthermore, the more you fill your frame with your subject (in your case, a human hand), the better your quality will be. That is, get right up close to that guy.
As well, the stiller that hand stays the better. A movement of 1/2" will ruin your model. So that rig sounds interesting. Just make sure you can remove it in the final model.
As far as scale goes, print out a scale sheet. I posted one on this forum recently that a student named Samantha Porter made. Put that under your hand. It has measurements written on it so when you get into Photoscan, just make some scale bars based on the sheet (it needs to be in your photos of course) and you're all set. Search my posts on this forum to find it or Google Samantha Porter. I can't remember what her website is. But she's the only Samantha Porter out there doing photogrammetry, of that I'm pretty sure.
That's my input. I'm fairly new to all of this but I have shot human faces and gotten admirable results. So it's not like I'm just guessing. There are many other people who could also comment that have far more experience and information than me. But no one had piped up yet when I saw your post so I figured I would.
Jon