I'll usually start my dwarves off as wrestlers, sometimes as early as the first migration wave, since they don't really need gear to train with (though it's always better to spar with armor). Preferably I'll give them shields and at least leather armor so they can raise up their other defensive skills.
Wrestling is always useful because it largely determines their chances of dodging, but once I switch them to weapons (especially crossbows) I have to be vigilant for any issues with their hands being full of grappled items. The 'Dump' order works fastest for that, and also for getting obsolete equipment out of circulation, because a hauler will immediately run up and yank the item off the soldier instead of waiting for their convenience. I have to check every now and then to make sure they're gaining marksdwarf skill, and if a dwarf just plain refuses to train at an archery range I'll make them a hammerdwarf.
If I want to get fancy, I'll also swap experienced marksdwarves to using hammers and let their hammer skill get quite high, then give them high-quality crossbows made of strong metals so they can whack the s**t out of things should it come to that. But again you have to be careful when you're mixing a dwarf between using ranged and melee attacks, because they can get stuck in melee mode where they only practice melee (even if they're carrying a crossbow).
I've recently heard people say that marksdwarves should be given bucklers [edit: I just finally tried this, it seems to work, they're wielding bucklers /and/ shooting at an archery range as I type this]. If this is true, then shield skill would be very very nice for them to have (a means to block would greatly assist their survival in archery duels). Dedicated marksdwarves are never given higher than Chain armor, because without Armor skill (or high strength) the would be terribly slow in Plate. A cross-trained marksdwarf (w/ hammer, shield, armor skills) is sometimes renamed a Sentinel.
Though marksdwarves are my earliest military units because they can kill enemies in a wide area at low skill / gear levels with less chance of dying, and there's a certain lethal prestige with having a dwarf that slaughters things in both ranged and melee combat, dedicated hammer dwarves are my real engines of destruction in the late game. They're ideally outfitted in full plate with chain armor giving extra torso protection, shields, and hammers (after they achieve high competency in Wrestling). I've seen well-trained hammerdwarves weave and block through hails of fire from enemy archer squads and terminate them one by one. A hammerdwarf's tendency to chase enemies as they're flying (dead) through the air actually helps them dodge arrow fire in my experience!
Even though marksdwarves are a bit more finicky to deal with (bolts scattered about, having to supply wood/bone bolts for training, wrestling issues or issues where the run into melee combat foolishly), I'd have trouble replacing them solely with hammerdwarves. Even a champion hammerdwarf can have a lot of trouble chasing kidnappers or routed enemies, and an elite bowmen who could be taken out of action with a single lucky bolt would inflict major casualties to a charging hammerdwarf squad. A mixture between heavily armored, shield-using melee dwarves (tanks in other words) and sniping marksdwarves is probably the most potent force that can be assembled in vanilla Dwarf Fortress.
As to organization, I'll only use squads in cases where I want to ensure a dwarf isn't patrolling a point alone. The squad's tendency to follow the leader means that a dwarf with 3 subordinates will usually either have 1 or 2 helpers to back him/her up, or will be safely back in the fort getting his/her drunk on. Off-duty dwarves are always, always kept solo so they can train at will.