1. what is the difference in interest between the quality of food? I guess the richer ones give more happy thoughts and can be sold for more? So... why produce the lowest quality ones? Because I don't think you produce less low quality meals for the same quantity of ingredients..
Growing a larger variety of food crops is mostly useful for roleplaying reasons ("I care about the quality and variety of catering at my fortress"). Even then, growing the inferior crops, like prickle berries, is virtually useless. Check on the wiki which food crops have a plant value of 4 and only grow those, with the exception of sun berries, because the drink made out of them is the best in the game. Note that sun berries aren't as readily available as most other crops, as the biome they grow in is rather specific.
It's good to have some variety in drinks, as otherwise dwarves will grow tired of drinking the "same old booze lately", but I think it's perfectly feasible to get by on just one food crop (I recommend sweet bots and quarry bushes; I'll explain later), as long as you also grow pig tails or rope reed for fiber, reserving some of the harvest for brewing. Ultimately, it's all up to you. I like to grow all underground crops, while focusing on my favorite sweet pods.
As for my reasons for recommending sweet pods and quarry bushes: the former, when processed to barrels to make dwarven syrup (this is important, as milling them to dwarven sugar works differently) and the latter, when processed to bags to make quarry bush leaves, will yield
more products than other crops and methods of processing. To be precise, the above give you 5 times the number of plants in a stack, while everything else gives you just 1 times that. To illustrate, a stack of 5 sweet pods, when milled will give you a stack of 5 sugar, but when processed to a barrel will give you a stack of 25(!) syrup, all in one barrel. Lavish meals made out of large stacks of dwarven syrup and or quarry bush leaves will be really valuable, especially if the cook is experienced.
Your priorities when raising livestock should be similar in that your decision what to raise should be determined by the modvalue and size of the animal. For example, a cow is more valuable than a horse, having the same size, slightly more fat, and a higher modvalue than it. Of course that doesn't mean that you should never use horses when cows are available, just that when you want to get the most bang for your buck, those are the things to look for.
One thing I haven't mentioned were preferences: when a dwarf likes a particular product, they will appreciate it more, obviously enough. However it is not feasible, unless you've got OCD or a very small fort, to cater to every dwarf's preferences, and going for the most valuable and/or abundant food is much easier. Besides, once your cook and brewer (notice the singular; just using one of each is preferable in that they will gain experience faster and should have no problem producing enough even for a hundred dwarves) become more experienced, the food and drink is going to be giving your dwarves good thoughts even if it's not their favorite.