Elves are not just humans with sharp eartops. They are the one of the eldest races in the world. But it is the only thing they can be proud of. Dwarves, they spit masterpieces of any crafts imagined for ages and millenias, Dragonborn build and rebuild their empire on the right of the strong. Elves sit in their forests, compose musics and admire starlight, concentrating on the fact how old they are, how unfair is the world around them and how primitive are the races who do not understand their high feelings.
Elves here are not more durable then men, just some minor changes in anatomy - blood vessels run alternative ways, some organs situated a bit higher or lower. All the same in total. That is why if a man thinks he'd hit a critical blood vessel or shot the liver, it may be not completely true.
Dragons and Dark elves truly have powerful mages, all of which ahve very high sicietal status.
Also, orcs will hardly believe that if Men won't wipe them out elves won't do either.
There is no way you can make a Dragonborn recruit. You can skulk and shamble through the world seeking for those on their spiritual development quests, if you want.
Dwarven mercenaries - OK. Dark elves - dunno, gonna see.
About mercenaries:
Well, you can ask for rumors of legendary ones and try to locate them, but it'll take lots of efforts, luck and money. And basically you may find a guild dedicated to this kind of people in each capital, which offers you wide assortment of different mercenaries they contract for a small fee.
The fact is, Dragonborn - those who are thinking in their
traditional way - are not very likely to have friends or foes. Or hate particular nations. there are individuals who they respect, and those who they do not respect. Pray to all gods you now not to be among the last group.
Four major races - men, dwarves, elves and dragonborn.
Also present, but their playability must be requestedand may be declined - gnomes, eladryns, goliaths, tieflings, halflings, orcs, goblins, trolls, drow, gnolls, cobolds, doppelhangeres, yuan-ti, shadar-kai. Last three are mythical and are almost a legend by now, but who knows...
A proper illustration I found for an occupied elven city with a Occupation HQ