The answer to the first isn't easy, it depends on a lot of factors, including size of map, projected size of empire, etc. Personally I try and find a flatlands area that has no "natural production" i.e. hills (later in the game almost anywhere can be made productive with workshops), but at least two if not 3 food resources. Then check that eventually irrigation can be chained to almost every tile. All you want in this city is food, nothing else. Flatlands jungle is often the most ideal for this (although just about every building must be built with slavery). This kind of city can often support 15 or more specialists post biology.
I'd again personally, say roughly 2.5 commerce to one production city, but a lot again depends on the terrain and your gameplay style. A warmongerer would probaly be 2 to 1 if not closer to 1.5 to one. Later using universal sufferage, even pure cottaged cities give a reasonable amount of production (+1 hammer for each town, thats 2 hammers and at very least 8 gold from a town on the flat / plains).
-- I did notice that its sometimes hard to track down these people in my nations/cities. I went far too many turns forgetting that I had a boat full of people to send to a remote land just sitting in my city!
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