As to building a granary:
Assume you have 5 excess food, and can thus pop out a settler every 8 turns with stable population, and assume you have the shields to do so.
If it takes 16 turns to build the granary (it will take less, but this is a simplifying assumption), that's 2 settlers before you get any, but you are then popping out settlers twice as fast - so you have 2 when before you would have 3, are tied at 4 and ahead after that.
It takes less than 16 turns to build the granary, of course - chopping trees help, and you will produce more shields at a higher level, but then, there is an advantage to having your second city out 12-16 turns earlier, so those things probably even out.
But that's not all - even when you run out of room to settler, the factory can *still* pump out worker after worker, for improvements or population growth in cities that you have set to, say, build military, and so have working 2 forests and aren't growing on their own.
I would be very suprised if the map is set up so you can't get more than 4 other cities - you might have to jam them in, but my guess is that you will be able to get 7-9 in if you are there with another civ. You might want to pop out one additional settler before building a granary, so someone can build some military to defend against barbs, for MP and for exploration.
What is an interesting question is: Can you build a 6 turner without a granary by irrigating cow and both wines? can you get enough shields to do that? I would think so, from size 5-7 - that makes for an interesting alternative.
oh - no barbarians. hmm. I'd almost bet that we are alone on this island. If it were us vs. 1 deity AI, we would get crushed without barbarians to distract them. And I dunno that we will have 2 AI on the same island as us on an arch map.