I'm actually with lolcats, i much prefer having a very low dwarf cap - around 20-30. The problem with migrants is that the vast majority are unskilled and there's no efficient way to train them, and then the other problem is that even if they are skilled there are only so many industries to commit them to. I usually just let a few migrants get good at the professions that I didn't bring with me (cloth, crafting of other materials than stone, etc) and then establish a small military (4-10), and the rest are haulers. I don't like having crazy hauler-worker ratios because too many of the dwarves go into idle mode; anything above maybe 3:2 is a problem for my fort because I hate seeing those wretched idler counts going up.
Unfortunately as a result, I find it really difficult to make use of more than around 20 dwarves; the industry system of the game doesn't really grow vertically (ie it doesn't become more and more advanced) beyond the first 10-15 skilled tradesdwarves. I can typically get my forges going as soon as I strike ore which is really just a matter of mining speed; and metal is pretty much the highest tier of industry that DF has. After that, any new dwarves that you add to the skilled worker pool are just doing more of the same stuff you're already doing which is only useful if you let more people join the fortress population which only extrapolates the problem. Vicious cycle much. Usually once I get my entire system going, my stocks of stuff will grow over the years and when that happens a few years in a row, I know it's time to go into megaproject mode (game is beginning to stagnate and the fortress is becoming over-stabilized). only then do I start accepting more dwarves, just to serve as construction slaves.
I do regret though that these low caps prevent me from enjoying a lot of the game's bigger features. Can't really get any interesting nobles or megabeasts at only 20 dwarves. Next time I gen a world, I'll reduce the attack timings to a population of around 30 or 40, so I start getting invaded as soon as I let my fortress expand beyond the comfort zone.
You are, of course, entitled to your own opinion, but i thought i might throw my 2urists at you; My last fort (abandoned last night due to a demon infestation ruining my fps even more than it was already) looked something like this:
Total pop: ~170
Profession: Number: Average level of relevant skills: Jobs:Farmers 8 8 growing/brewing/cooking
Farmhands 8 8 all farming except brewing/cooking
Craftdwarves 6 10 wood/stone/leather/bone crafting
Smelters 6 15 furnace operating
Metalworkers 3 9 weapon/armor/metal/blacksmithing
Stoneworkers 7 10 mining/masonry/engraving
Woodworkers 3 8 carpentry/woodcutting
That was with about 60 haulers, 30 marksdwarves and 4 axelords, and a few other random things (children, mostly). I maintained <10 idlers for that entire fort. My point being, industry definitely does scale, and when you have a large fort, it's quite easy to have a large number of workers, all of which level up to legendary in at least something they're doing within 3-10 years. There's plenty to be done with so many dwarves to satisfy, especially when you're being attacked every other season.
In terms of the most efficient way to train them, it might seem obvious but my advice would be to build plenty of workshops, turn off hauling and kick their lazy ass into gear