Would it be detrimental to have, say, 2 spearmen, 3 hammermen, and 5 axemen in a squad, since they would train each other in skills that they aren't going to use?
They wont do that. I tested it, and they only attend demonstrations for equipment they actually have equipped. However beware of wafflers. If you set the weapon to "individual choice" in the uniform (and have multiple types of weapons available in the fort) then the recruits will waffle endlessly over what weapon to use and will never gain any real skill in any of them. Assign them a specific weapon or weapon type. This isn't so much a problem with dorfs who already have high proficiency in a weapon skill.
However it's always best to train in squads of two, because that maximizes sparring. A squad of 2 dwarves, will soon be more powerful than a squad of 10 dwarves, because 2 ultra-legendary dwarves beat 10 average joe dwarves. Give it a try by embarking with 2 profecient weapon users, and let them train with each other - and no-one else - for the whole game. They will become like gods.
What are the different perks of each weapon type?
Axes - Cutting off limbs and heads.
Spears - Stabbing organs, also good for stabbing heads (when an enemy passes out from pain, the spear dwarf will immediately put out their lights with a spear through the head), good armor-piercing ability.
Swords - A decent compromise. While some don't like then, I've been nothing but happy with my sword dwarfs.
Hammers - A good choice if you only have copper or silver.
Maces - Better described as a meat tenderizer than an actual weapon. I had a mace dwarf bashing an ogre or something in the head, shattering the skull on every hit, but not actually killing the poor thing. Just pages and pages of the beast having it's skull shattered. Don't use these for real combat.
Picks - Effective at removing limbs and punching through armor. However the pick-warrior will always be called a wrestler since picks use the mining skill. This doesn't effect their combat-effectiveness though.
What's a good way to crosstrain soldiers and how long does crosstraining take?
Don't, the sole exception being training miners to use as pick-wielders. Dwarves gain attributes MUCH quicker by sparring in small squads than they ever could be cross-training.
However there's nothing wrong with having full squads of marksdwarves, who are kept permanently inactive (and without a place to train at) and allowed to haul and perform other duties. When you need them, or want to train them by shooting at live targets (which is highly effective), just station them and they automatically activate. When you cancel the station command they return to their civilian lives. But the cross-training wont be worth much at all, so just let them haul or otherwise help out.
What's the best way to hold back the evils of the caverns below? Have a barracks between the entrance to the cavern and your stairway?
If you're not going to seal off the caverns, then a barracks in the cavern or on the stairway is a good choice. However forgotten beasts will rape your militia if you don't have a plan for safely disposing of them - killing them isn't the problem, the problem is the syndromes will kill your dwarfs afterwards. One solution is to force FB's to meet your militia in a checkerboard pattern arena. The diagonals don't allow gasses to pass through, but do allow dwarves to attack diagonally. This will reduce the militia-rape from vapors and such.
How can you employ magma and water as methods of defense for your fortress? Could someone explain to me and the other Dabbling Dwarf Fortressers[V. Rusty] the intricacies of the almighty pressure plate?
I don't recommend fully automated drowning traps. That's asking for trouble. You can have a drowning chamber which automatically seals, but activate the water/lava using a manual lever.
The most foolproof way to use pressure plates to automatically seal entrances is to use retracting bridges over ramps. So the entrance should be a ramp going down. A retracting bridge is built which seals the hole. That retracting bridge should then be linked up to a pressure plate which is set to be triggered by friendlies, and to minimum weight. Create a 1x1 pasture on that pressure plate and pasture a single animal to it - anything which isn't too small will work. The animal will hold down the pressure plate, which will hold the retracting bridge open. When invaders come, the animal will either get spooked and run, or it will charge. In either case the pressure plate is then released and the bridges return to their default state - sealing the entrance.
There's a few reason to prefer this configuration:
1) A retracting bridge over a ramp/empty space cannot be blocked from closing.
2) A retracting bridge cannot accidentely squash friendlies.
3) The pressure plate with an animal on top functions as a "dead man's switch", the default state is lock-down. Absolutely anything capable of killing the animal or making it move will trigger lock-down.
You can also easily install this system to seal corridors automatically.
++++>║<+++++
++++>║<+++++Have a set of ramps which goes under or over a wall built across the corridor.
Build a retracting bridge over one of the set of ramps.
Construct the "dead man's switch" pressure plate setup on whichever side you find most useful.
Automatically sealing stairwells is even easier. A retracting bridge can be built over up/down stairs, and will prevent anything going downstairs while closed - but the upstairs will always work.
My preference is to use these dead-man switch setups to lock the enemy outside with my seasoned military - the barracks are *outside* the locked down areas. Although rookies may be allowed to train deep in the safety of the fortress.