Consider me duly slapped. (Hmm, I enjoyed that ...)
I guess the thing to take away from this is to ask the printer what they require. Each seems to have different requirements, depending on what kinds of machines they use, what sorts of things they print, and what kind of paper they use. I had a stint a few years back using printers and this point was continually drilled into me.
As for the OP's other question about font sizes, it again depends on what kind of machines the printer uses, so ask them what's too small for them. Some really high-quality printers (for the really fancy print publications) can print very fine text accurately, but not all can. You've also got to consider the difficultly of a background color with colored print over the top. Some print machines print each color separately, so the background may be printed first, leaving space for the text to be printed next. Getting that to happen accurately is difficult, as there is usually some smudging (not the correct term, I know). The type of paper being used is also a factor. The more absorbent it is, the more the colors will run, resulting in a muddy mess.