It helps to know that they're coming in advance... you really need to have a carpenter on repeat-build beds, a stoneworker on repeat-build doors, and masons digging out as many rooms as possible, all with no other tasks on their jobs list whatsoever. You can use a barracks, yes, but I recommend avoiding this... Individual barebones rooms are easy to churn out once you get the hang of it and give
fourfold the happiness gain, plus they let you make dwarves even happier by improving them and putting neat items in them. All of this happiness will buy you a lot more leeway if other problems crop up, even with a food shortage.
Anyway, everyone else should focusing on reaching the river and setting up a huge farm right from the get-go. Of course, a river entrance can be a little dangerous, but you can minimize this by arranging things so that the only part of your fortress touching the river is a channel, and by having several tightly-closed doors and traps between this area and the rest of your fortress.
There is no substitute for a farm. None. Fishing can keep you alive for a bit in an emergency, but try to avoid depending on it as much as possible; it's so inefficient compared to a farm that you're basically just throwing your time away... not to mention the fact that fish need to be cleaned and rivers eventually run low on fish.
(I do recommend having a single fisherman and cleaner even later on, though, just to produce bones and shells for your craftsmen to practice on.)
Anyway, it's not to late to start on all these things now, especially if you put all your new dwarves on it, but you should hurry.
[ August 14, 2006: Message edited by: Aquillion ]