Is Gotham 6pt legible in a printed magazine? What is considered to be the best font size for printed editions? Is it ok to use different font sizes on different spreads?
It depends which Gotham weight you'll be using and also are you using the 6pt size for body text or smaller copy (eg. footnotes, etc)? Gotham Book which is the "regular" weight should work in 6pt but you can also consider going up to 8pt for body text. It is generally a matter of personal choice and it all depends on your volume of text, experience with the font family, etc.
It is also a good practice to print a few pages of your artwork on paper to see what it looks like in reality. This can be done via a regular black and white laser printer or whatever you have available.
Hope this helps