1) Only in so much as higher class can afford more farms

2) 1 food = 1 population

3) See 2.

4) I usually go 1 hospital, 1 farm and 2 approval buildings off the bat, then add more on bigger planets. You'd have to do the math, but you'd generally want more farms / entertainment as you tech up and your % modifiers gets larger, especially from multiple starbases.

Np

Note that you pretty much have to do the calculations on the fly since every planet is different, but you get a feel for it eventually.

If I recall correctly, the exponential function breaks even' at 20 pop = 20 raw production, so I like to use that as my baseline. If another farm gives, say 4 food (depending on tech and modifiers) and you are at 20 pop, that's about 20% increase in raw production (more like 15% because of base production). Say you have 400% modifier to production from buildings, that means another farm is better than a building giving 80% modifier (20% of 400%).

At 60 pop, the same farm would only give about 3 production (from 45 to 48), so about 7% bonus. That means if you have the same 400% production bonus, you'd only need a building giving 28% modifier to be better than another farm -- so obviously only a huge planet with a big network of bonuses should get 60+ pop!

On the other hand, a single farm is almost always worth it.. going from 5 (base) to 9 pop gives you almost +50% base production.

Edit: Oh, and try to get up at least 4 star bases around each bigger planet as the +40% to base production skews the calculations heavily in favor of getting more pop. Don't worry too much about the micro management of upgrading the star bases, that's really only necessary on huge planets.. the base production is the big one you want.

You also get +25% raw production if you are at 100% morale.

if you factor in the 5 base production from the colony capital, it's closer to the previously mentioned 20 pop - 20 prod. but if you do that, the "production per pop" curve is also much flatter and small planets with little to no farms are more efficient. 

my rule of thumb is that i only grow the population if i can get enough approval to keep them at 100% WITHOUT entertainment buildings on the planet. if you have to plop down a stadium for every farm, there aren't that many cases where those two buildings are actually better than 2 factories or 2 labs.

No, the population contribution is 2 * (population)^0.7.