I got another wave of immigrants, now up to 55, and immediately I ran out of food and drink.
Doesn't sound wrongly misbalanced, everything you put. Do you have military guys drawing food rations? They tend to bite claim something, bite on it and then go claim something else. I set the military to carry drink (need flask/waterskin) but no food. Let them eat from the regular dining areas (every now and then) but carry their drink around with them. Or that might not be your issue (or how others deal with it).
I've had at least one fisher and one hunter (who's now out of ammo) for most of the game, but I guess they're not very good :-)
I don't use hunters/fishers, these days. Get a breeding pair of any fortress animals, wait a short time, and you've got more. Check the attributes of any spare adults you have of a species and set to butcher them. Make sure you can haul the stuff out of the Butcher's shop quick enough that a) The bits that rot don't rot; b) The bits that don't rot don't clutter it up and slow down the next butchering job. Let the kitchen cook any meat items you get from this (I disabled booze cookng, seed cooking and plant cooking, to preserve booze, seeds and seeds-in-potentia; I probably also disable milk-cooking and set it to being made into cheese, which I
will let be cooked). Prepared meals are better than the sum of their parts, generally.
As to what a kitchen does when it's not preparing meals? Rendering fat, isn't it? If you have fat (which you don't, if you're not (successfully) hunting or butchering as above) then you can make it into tallow. Which has uses. Anything else kitchens do isn't coming to mind, without the game (or a window on the Wiki) open in front of me.
Obviously you need the spare food items (consider also grinding cave wheat/etc?) to go for prepared food. Doing a two-item snack thingummy, whatever it's called again, is good to build up the Cooking skill for minimum ingredients (last time I checked), but once you've got a decent skill then you get better value/more meals per action/more satisfying meals overall if you go for the 4-item roast thingummy.
When you've got spare food items to cook with.
Your farming looks sufficient (I've seen recent calculations that a small amount of farms (possibly a couple of 2x2s but certainly less than a single 3x3 for each crop-type) is capable of supporting a 200-dwarf fortress, but I can't remember the precise details), and probably just needs tuning somehow, in a way I can't bring to mind. You see my forum username there, to the left and up a bit? Well, there's a reason I chose that. I took a
long time to work out farming (and food in particular). What really helped me was a fully-aquifered map with all but one non-soil ground square warning as wet, and so I spent some time dedicating myself to a soil-based fort and a lot of farming. Since then I've (barring oversights, or rushing to the magma and forgetting farms until I was already near starving) generally been Ok. As to
what I learnt? Darn hard to describe, to be honest. Practice?
Workshops - Is it one dwarf per workshop? I don't remember seeing that, but might explain the long queues.
Yes. One at a time, with current Workshops (possible future room-designation-type workshops might differ, and allow multiple 'workstations'). It's
at most one dwarf per workshop, in fact. You can also have long queueueueueues when a workshop isn't currently being worked in. (Dwarf may be on way there, or off to get something to eat, and no other dwarf is stepping in to work on the job he's not currently doing. Or just everyone's off sleeping/eating/drinking/whatever.) Again, hard to describe what I'd do, to get things done faster (save for making more workshops and dedicating more dwarves to the job-specific allocation in the hope that they'll jump in there and keep things going), but it'll be a matter of fine-tuning of some kind. And practice, again.
Fun! I've just mined down into the middle of a cavern. I was going to put an up/down stair on the cavern floor and fortify walls around it, but I cannot seem to designate stairs (it's only one level down). I have instead put down an alternate stair into the last mine room, and wonder if I can destroy the stair that drops into the cavern, replacing it with a hatch?
If you've dug into the ceiling of a cavern then (as long as you have an orthogonal spot for a dwarf to stand), you can Construct an up-stair over the final up/down stair, and it will be a pure-up stair and flier-sealed. Unless a dwarf is actually
falling down the stairs (which I've heard happens, perhaps in combat on a stairwell? ..but I've
never seen it happen myself), there's no problem having an 'open-bottomed stairwell' for normal purposes, but sealing it to prevent flying cavern inhabitants from randomly wandering in is always useful.
(Or you can put a hatch in there over this 'loose' up/down, locked or otherwise, and I
believe it'll still operate as an Up.)
Now you see the cavern, you can work out where to side-step and send stairs further down. I'd advise doing so in the middle of a wall, not at the edge of one, so that you don't have a walkable breach into (and, more importantly, out of) the cavern just yet. But that's just how I work and you could breach the cavern and then work on isolating an area if you want.
Regarding military defence against the Giant Bat, it's generally a matter of using the Squad menu to designate an attack (either by cursor on a tile/area, or a specific creature) and see how it goes. Darnit, another thing I know how to do when the game's in front of me, but can't describe. Fat lot of good I am, ain't I?
(OTOH, unless you have a walkable path, flying beasts will not (in
my experience) deliberately path through fly-only paths into your fortress... But as they randomly wander (and fly) around they do seem to find such entryways anyway. I'm not sure how much is Drunkard's Walk chance and how much is a little devious bias to...)