I have already built more upscale structures. Is there anyway I can feed my people without farms? Can restaurants and markets completely replace farms?

Ahhh ... the wondrous fishing wharf! (I presume 'ranches' count as 'farms'?)
The deal is that restaurants, markets, and army bases act as distributors of food - not creators of food.
To digress slightly; A pineapple is grown. It is stored at the farm until (a.) the farm burns down by rebel attack, or is demolished by El Presidente, or knocked over by Hurricane Harry, (b.) someone walks up to the farm gate and eats it, (c.) a teamster or a farmer takes it to said 'restaurant, market, or army base', (d.) a teamster or a farmer takes it to the docks for export - presumably the teamster's office and/or docks aren't destroyed somehow in transit, or (e.) it is taken to a cannery for further processing for export - presumably the cannery isn't closed down / destroyed.
Oh. Just to add to my query: How do I decrease the population after I have built my isalnd? I currently have 900 denizens in my island. For now I need them but once I have already built my island, I want to decrease my population to about 200 plus. IS this possible?
Elimination edict? Mwah ha ha!

Try 'Love it or leave it' with your immigration office as a first step.
You can try to hassle people into leaving by making their lives miserable, but this taunts the military or rebels into rather unpatriotic acts. An uprising is a pretty good way of reducing the population too!