Important note: Get a full-steel and silver war hammer squad at the ready as well, or else zombies with a necromancer would kill you, adamantine or not.
As mentioned previously, 1-2 squads of crossbowdwarves is recommended, maybe elect to give each an adamantine helm or mail shirt. Give them copper crossbows, copper shields, and steel bolts. Each dwarf should have, in a perfect squad: a leather armour, an adamantine mail shirt, an adamantine helm, a copper shield, a copper crossbow, five leather hoods, five leather cloaks, a leather quiver, steel bolts, leather leggings, leather high boots, leather gloves, leather mittens.
This will give them good enough protection to last until a melee dwarf can retarget to their aggressor, and potentially, with enough hammer dwarf skill, maybe kill a goblin or two once their ammo is out. Against demons, keep these guys behind fortifications.
Another special note: keep all military dwarves from woodcutting, mining, and hunting labours, as this will strip them of their uniform every time they're off duty. Think of giving your legendary smiths and other important, non-military dwarves a full set of steel armour, complete with two copper shields. If invaders get through, order your legendaries to a safe zone. Maybe post 2-3 crossbowdwarves with them, or give a retired hunter a role in that squad, as escort. Keep these guys away from fighting.
Another note: The fortress guard should be crossbowdwarves, armed with wooden crossbows and shields, but following the same setup as the crossbow squads mentioned above. This makes them give weak beatings, but still remain efficient fighters in an invasion.
Another, challenging note: If you have the time, resources, and the drive, put everyone in a militia. This gives everyone basic protection, and keeps all but miners, hunters, and wood cutters safer at all times.
A militia uniform would consist of: 1 wooden shield, 1 leather armour, 1 bronze mail shirt, 1 bronze helmet, 1 weapon of either a bronze cutting weapon or silver hammer, leather leggings, leather gloves and mittens, 1 leather cloak and hood, leather high boots.
Maybe put each squad on a rotating training time, one month a year, each squad having a different training month. They'd be light enough to not lose any speed, and could fight back against any goblin who comes charging at them.
This leaves woodcutters, miners, and hunters pretty much the squishy meat shields. A legendary miner, though, is deadly with a pick.
Follow this set up for your military, and your fort will be ready for any attack. I used a similar setup to fight hell, and actually beat it, with only losing 19 dwarves out of 120.