I would turn immigration down completely, is there anyway I can... Encourage my dorfs to breed?
In fact I am preparing right now, I genned of the best worlds I've ever seen.
Can I take birds? They would be a steady source of leather, bone, meat and eggs, but I don't know if that's against the rules of the challenge.
Just mod certain cave creature to be tameable. Big, herdlike things such as Elk Bird, Dralatha, Drunian, and Rutherer work well. Naked Mole Dogs are tough and easy to trap, but die in ~4 years. Good news: if you get too many, just cage them and wait.
I actually made some avaliable on embark:
Elk Bird: replaces most livestock
Naked Mole Dog: replaces dog
Large Rat: follows dwarves around like cat, but doesn't kill vermin
Creepy Crawler: work in progress. I changed the name to "Large centipede", because that's how I imagine them. They can be tamed, and kill vermin like cats. I'm trying to make it so that they can't adopt or be adopted as pets. Wandering, scavenging rat-eaters.
Most actually cost a shit-ton to embark with.
Don't kill the Giant Olm. They're fast, strong, and have fairly tough skin. Wait until you get better soldiers. Or, try to trap the thing, tame it (mod the raws), and keep it as a pet. I got one and assigned it to my Fortress Guard Captain as a pet/guard.