Thanks for your answers, it's a great help!
1) Are Great Prophets better for food bonuses or are any GPs OK?
2) How do you balance between selling food and holding onto food for population growth?
I don't want to thread jack, but what I do is I keep a good eye on the market to see what's going. I'm at 15, but I think the second biggest city is 10 or 11. Also, I've been ahead in the cash game for a long long time, like, I was at 15k when everybody was at 1k or so. I won two early "convert 20% of gold to shields", the second one gave me about 10k shields, so I never needed buying shields, and I have ginormous granaries everywhere. In short, I never needed money -- so I sell when I feel the price is high enough, regardless of growth. And when the prices is low, I buy food back. This might vary game to game, but for this current game, I was able to keep food at a consistent 400$ early on, and if anybody mass sells, I buy them all up to 400$.
I pop a citizen when/if I have enough harvests that after popping I have ~10 harvests left. This way, if someone wants to grow their city and buys their food in a bundle and drives the price up to say, 900, I can immediately harvests a few times and bring home the cash.
The trickle probably adds to the math (I didn't realize there was a trickle until I visited this forum, which was after I had my strategy anyways) but the additional guy, unless you have tons of great prophet in the beginning is not dramatic.
The key thing I do is that I have way more than enough money to run out this scheme, because I can be fluid -- I can buy all that I want when it's cheap, and I can sell all that I want when it's expensive.
Alright, it got a bit wordy, I guess the
tl;dr is my current strategy involves buying food when it's cheap, and selling food when it's expensive. I pop when popping still allows me to buy food when it's cheap, and sell food when it's expensive.
Edit: I missed your first question. Only Great Prophets is for food. I really keep buying them, and maybe only used one or two for wonders. I also buy other GPs when they're cheap and sell them when I make a little more than break even, and sometimes use a few to build wonders. I didn't know about the trickle though, so right now I just buy other GPs when they're cheap and working out the math. (The math for me is much better for farming because I work the math into the harvest)