Use a comma if the appositive is after the subject (and note no capitalization of the title)," i.e. "John Smith, president of Shmo Inc." However, if the appositive is before the subject, no comma -- "President of Shmo Inc. John Smith." Same capitalization rules apply when used in the middle of a sentence.
I don't even understand your other question. Commas and apostrophes are not interchangeable.
The only comma-rule I'm sure about is an apposition (Emma, who is a beautiful girl, loves John). Appart from that it's only guessing but I actually need that knowledge. I hoped some cool Freeper could tell me on ten lines where to use it and where not. Otherwise I'll just buy a grammar.
My question about the apostrophe was if I use it in the genitive (floridarolf's Coke or floridarolfs Coke).
"Ac centuate the Positive
E-liminate the Negative
Latch on to the Affirmative
and come out with Mister in between"