I should preface this by saying I know very little about the military . . . I think I got a couple of dwarves to spar together once. ONCE.
1 ) What % of my population should be in the military?
Depends on what else your dwarves are going to be doing. Crafting mugs? Building a megaproject? Standing around? Considering how ridiculously productive farming is, and how ludicrously expensive prepared meals are, you could theoretically get by with a
single Grower/Brewer/Cook, and put everyone else in uniform. Alternatively, if you're building a Borg Cube that covers the map and have easy access to steel, you could possibly go with an Army of One.
2 ) What are some good ways to train my military en mass that doesn't feel superbly over powered?
Above, I said I don't know how to get them to spar. That's because I'm a fan of mass pitting. Just set up an
insane amount of cage traps in your very long entrance hall, drop the invaders on your army, done. (Tips: Put a door outside your hallway, to keep wildlife from cluttering up your traps. Make sure to mass {forbid} the traps & cages when there are any hostiles about.) Now, cage traps
are overpowered, there's no denying it; you can mitigate this by only using a couple of them, so you can pretend that goblins actually ARE smart enough to stop advancing when they realize that the entire hallway is trapped. But even if you capture only 1 or 2 per season, you can still disarm them, and toss them into a pit with a dwarf with a wooden weapon: He'll take quite a while to beat them to death with only a training sword, which means lots of experience for him.
3 ) What is a decent fatality rate?
I honestly wouldn't know, my current fort is 24ish years old and has yet to see worse than a bruise. I haven't even bothered to set up a hospital.
4 ) Should I restrict my military to only male to avoid infant deaths?
I'll be restricting married females from my melee squads, once I make them. It's not just the babies they carry, but the children that tag along behind them. Y'all can just stay behind the battlements, thank you very much.
5 ) Is it worth waiting for steel production, or should I just pump out some Iron/other metals in the beginning to get off my feet?
Again, I wouldn't know, but I would echo the others in saying Use What You Got.
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My own style is the Miner-first, abuse cage traps, make steel almost immediately, Army of One approach. All of my starting 7 quickly became Legendary Miners, my expedition leader got a helpful mood which leveled him up even more, and then I encrusted him with ☼steel everything☼ and tossed him at the first megabeast that came along: A jungle titan made of sand. Then I locked him in a room & dropped lots of disarmed (but armored) goblins on him. He got tired of slaughtering foes that couldn't really fight back (and named his pick "Swordamaze the Withered Zeal" to illustrate that he didn't want to fight any more battles that lacked honor), so now I only drop large groups of trolls on him--he slew 51 of them last time. He's now the duke, with over 230 notable kills, including the dragon that he dueled outside the gates, dropping it in 2 hits as the moat burned around them. Didn't even singe his beard.
Naturally, an Army of One is very risky, as the rawest bowgoblin recruit can still one-shot
anyone if they roll a Critical Hit. But since I've already savescummed about 1000 times on this fort, what's a few more? My duke has been shot, hammered, gored, bled, and ripped limb from limb. If savescumming isn't your style, don't go with a single soldier--or even a single squad, as they could, for instance, all fall prey to the same FB beath/webs/syndrome and leave you helpless against whatever comes next. If you're serious about defense, have a "citadel" with its own inner defenses, and its own separate guards.