If you have more migrants than jobs, assign hordes of them to tasks like wood burning and furnace operating, anything where skill doesn't affect the final product's quality. if you still have too many, give them all masonry and start building something huge.
Don't let any migrants work on your farms unless they have high grower skill, unskilled growers will cause crops to produce less. Brewing however, can be done by a team of unskilled workers, as booze never has a quality level.
EDIT: Just finished reading page two, looking at that !!science!! thread now.
EDIT2: Read it, still I seem to notice more legendary drinks with a legendary brewer, though that doesn't mean they are wrong as I probably haven't looked as hard as the OP did in that thread.
I inevitably end up running a No Job party fortress to my chagrin, which probably explains more migrants after the carnage that has usually transpired in the past. (Though that's just the way I like to think it works)
One thing about migrants is that they arrive with hauling labors enabled, so generally the party begins or it's interrupted by mass designating a room of rocks to be dumped in the stoneworking room near the workshops. I tend to use designate instead of using stockpiles in this instance because all the idlers with hauling enabled will grab a stone and take it to the dump spot in the stoneworking room without pausing for a full stockpile.
I tend to counter not having a job for all the dwarves by setting max population low in the init, though sometimes that means not enough dwarves, and it's a long wait till more have grown up. However I think you can change the max population in the init at will for more migrants after reloading, though I may be wrong as I usually wait for attrition to cause a gigantic migrant wave once it's below the threshold. Sometimes I end up one death away and with things everywhere calling desperately for a dwarf to haul them, and then some happy dwarf family has twins or something.