I do this a lot, and the best noise generator I have come across is Camera RAW > fx > Grain. It is quite a game changer. You have lots of control, but I generally go with the defaults and only adjust the size according to the resolution and how noisy the original image is. The low teens works best in my experience.
Then there are things like skin tones which need a bigger and more irregular noise pattern. For that I use the scatter brushes in the Hair & Skin Textures by Castrochew brush set. Check out the arm and shoulder in this digital painting I did of my wife, (Note: yopu can view it at up to 2000 pixels high). That area looks close to photo-realistic. I apply the noise texture to a new layer (as you would) and set the opacity so you can barely see it (about 5%).
You can use the 50% grey layer set to Overlay to add noise non destructively, but that doesn't give you the control over colour.
The trouble with that route is I dont have the picture in with the grain to visually match it to. I have to have the RAW window overlapping the target photo and both at same magnification, but then as you say low teens on size, the only slider lacking though is contrast, as the difference in tone between the darker and lighter grain I wanted to reduce, roughness one would think controls the amount of tooth, like fine and coarse emery paper, but didnt affect this shadows highlights effect. I guess further refinement in photoshop would do that, or maybe in raw controls basics. That said I then did another 12 25 50 ans then went to raw basics but found contrast slider simple made everything go lighter, not lessen the tone between the lighter and darker grain, tried black and highlight sliders, no joy. Which settings to lessen the tones and bring the grain differences nearer each other ?
I seem to recall a way of selecting part of a photo then using its grain to make a solid colour the same as the grain selected ?
Yes, it is possible to make a seamless repeating pattern from existing grain in an image if you have a nice large flat area to borrow (the larger the pattern fill area the better in my opinion).
An old, but still very good example can be found at the following link (which is not about grain/noise, however the technique still applies):
That being said, I generally use synthetic noise/grain methods.