I *think* too-high temperature was mentioned as being bad with gelatin because it harms the gelatin somehow, but I have ran 320ppm close to boiling temp without adverse effects. It should be fine.
You can make Colloidal Silver cold. This is not just limited to 20 PPM. I do 100 PPM cinnamon-reduced cold because cinnamon is an extremely reliable and quick reducer. And I tend to get less turbidity when limiting temperature. Except for gel-capped where it doesn't seem to have a bad effect.
You can easily tell if particle size is bad. Color will be shifted into the green spectrum. As long as your Colloidal Silver is greenish-yellow to yellow to orange yellow, particle size is in an okay range. Obviously yellow is ideal. I have not been able to produce Colloidal Silver with too small particles no matter the temperature and which other variables changed. I would be surprised if you were able to achieve dangerously small particle size easily or by accident. It doesn't seem to usually occur.