On the topic of armor, it's generally best to just start with a mail shirt and helmet then slowly upgrade the armor as they get more skilled in armor user, and can handle the additional weight. At legendary armor user they could wear full slade armor and not be slowed down the slightest, so there's little point in intentionally lightly armoring skilled dwarves. Though notably, mail shirts are actually heavier than breastplates. Not by much, but they are a bit heavier if you ever check the numbers on them. I still think they're best for a lightly armored dwarf though because they cover significantly more area which is quite helpful when you can't armor everything. On a fully skilled dwarf it doesn't have to be a choice between chain mail and breastplate though, they can equip both. They can even equip 3 mail shirts and a breastplate.
Basically this. You don't start the dwarf in full plate with 3 chain shirts and 7 cloaks... as its actually an ENCUMBRANCE penalty. This doesn't just gimp their speed, but it gimps both their attack speed and endurance. So if you over armor a dwarf he both attacks less and passes out sooner. Its a huge mistake.
I'd also add that you -must- give your dorf a shield as well as that chain mail+helm. The shield is the single most important piece of armor, probably followed by that (light weight) helm. Then you can fill in the rest with leather and bone. They should be light weight enough while still giving some protection... and more importantly armor user points. Armor user will level more quickly the more pieces you have on. So again, start with the gimp armor and then add more "real" pieces as the armor user level increases. I think at 5 they can handle a "full" simple set of boots/greaves/mail/plate/gauntlets/helm- all of metal. At level 0, they can, at most, handle one "big" piece of metal armor (mail shirt) and then only light pieces after that.
Also, I hate to say this, but 4 military dwarfs is a laughably small military for any fort outside the first year. 11 dorfs is also next to nothing. A single FB with a syndrome or webbing can wipe that out. It can probably handle smaller sieges no problem if they have great equipment, though. What you really need to do is break up that initial squad, and use every member as a captain of a new squad with fresh recruits assigned the same weapon as the veteran captain. His high level military skills and teacher/organizer (from demonstrations) should greatly speed up the training of the rookies. If you are worried about sparring... you don't need squads of just 3. You can make a squad of 10 and just edit their orders to have 5 separate "train minimum 2" order to promote multiple sparring matches in the same barracks.
Also... yes it is necessary. As I mentioned early, a single "oopsie" can leave your small military crippled and your fort undefended. There is just so little work to actually be done in a fortress, as well. So after about 30-50 dwarfs dedicated to growing food, hauling, and creating masterworks... what do you do with the other 150? Military usually. Now that we have libraries, we can split the excess between scholars and soldiers (personally I turn most of my soldier into scholars during peacetime, as it makes for great cross-training with the teacher/student skills). So unless you are playing with a very small unit cap... its not such a question of "is this necessary" as "why the hell not? They aren't doing anything useful anyways..."
Oh.. and when it comes to weapon type... I'd say hammers and axes for your "go-to" weapons. Steel hammers are actually comparable to silver ones, so it depends on materials available (generally, if you have silver spare yourself the trouble of steel-making). Hammers are great for killing armored units, as they'll crush someone's skull through the helm. Axes are better are severing limbs than swords (bigger contact area), but swordmasters can usually sever limbs of human-sized opponents (axe-users sever more limbs at a lower level, though). Axes are also heavier than swords, which -does- matter even to edged weapons. If an edged attack fails to penetrate armor, its damage is converted into blunt and passed through to the next armor layer. This is why ultra-lightweight candy bolts are laughably ineffective... iirc they barely worked better than wooden bolts.
Spears are good at one-hit kills, but have a high chance of getting stuck inside a unit and need to be pulled out. For a main-weapon, they are ineffective. Even if they go straight for the heart, they can get stuck and need a combat round or two to yoink out. Also, most weapon types in the hands of a legendary user can one-hit kill. The main advantage of the spear is dealing with large beasts. Think of a hydra. Its got 9 heads. Chopping off 8 of them still won't instantly-kill the hydra. However... it only has one heart. A sword, axe, or hammer won't be able to reach the heart through all the flesh of the behemoth. A spear will have enough penetration to skip straight to the juicy bits.
I just don't like swords, tbh. They feel like the generic, bland, vanilla "jack of all trades" weapon that is outclassed by any other type in terms of performance. Can't penetrate like a spear. Less lethal than an axe against unarmored units. Less lethal than a hammer against armored units. Even with the new undead changes I'd rather use axes than swords. However, swords and axes both have multiple attack types. Both include "flat slaps" and "pommel strikes," which are ineffective blunt attacks. Hammers just smash- there are no lesser attacks the dorf might decided to use mid-combat. So generally, I just use war-hammers as my main weapon with at least a squad (or two) of spear users to bring out against FBs/Megabests. Oh... and swords just seem too much of a "human" weapon. I always imagine dwarves with axes or hammers :p
Crossbows... are finicky. They are still good to leave behind fortifications, but their main use is breaking up enemy ranks. Have them behind your melee, and the bolts will momentarily "stun" the attackers as they have to dodge away, grunt in pain, etc. This helps prevent your main forces from being overwhelmed. They also have the distinction of not needing metal. So if iron or even copper is tight, you can make wood/bone crossbow teams with bone bolts. The bone bolts won't be the most lethal, but they can still cause some faltering of enemy advances.