Ha, yeah.
My first few fortresses also fell into ruins quickly, mostly due to starvation and snakemen attacks and less due to floods and the like.
I'm still not super-awesome, but that's mostly because I don't have enough time to get everything done, but I can definitely make a fortress the will survive indefinitely (theoretically), and already saw all of the major hallmarks of the fort.
So I can definitely tell you that things get much better the more you play and improve.
As for crafting, don't waste light or dark stone on crafting. Obsidian short swords are currently exactly like steel short swords, so you don't want to waste obsidian by turning it into some toy boat. You also need the limestone for making steel stuff. And while we're on the subject of weaponry, give your new recruits the crappiest weapons possible (copper weapons, mostly), since when they spar the can injure each other, and you don't want them to kill each other. This makes Obsidian short swords awesome as weapon trap components.
As for immigrants, don't be too surprised if you have more than 25 dwarves after the spring immigration wave.
The wiki states that immigration depends on the created wealth of the fortress, but I think that other things affect it as well (like the number of dug out squares).
Anyway, the best advice for running a fortress is "Check the wiki, constantly."
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