200 is my highest, always wanted to try more, but always knew that that'd just be lagmongering. FPS was around 30, with a very flat fortress. Most of them (4 out of 5) were usually busy, lots of metalworking, butchery (and tanning and cooking), mining, engraving, gem cutting, hauling of course...and large plantations. Also about four full military squads.
Idling isn't so bad, really. It means all jobs are getting done, and you have free hands to assign. When you have lots of idlers, check their qualifications and ramp up the volume in those industries. If they're untrained in anything, check what industries of yours are shortest on labour and just make them train in those. And if you really fail at such management tactics, you can always just toss them into the military.
Generally speaking, you want to have at least basic commodities for all of them, and a sufficiently oversized food industry. Excess stocks can be sold in return for rare items or military equipment. Violent threats shouldn't be a problem, since your military WILL be of sufficient size. All you have to fear is resource misdistribution or tantrum spirals - so as said, try to manage your industries properly, and make sure your dwarves have everything they need to be happy.
If you find fortress layouts to be difficult to make for high populations, just make several forts in a single site. Maybe one for each biome if your embark is large, or spread them over the various z-levels otherwise.
Should you have FPS troubles (predictably), the thread on pathfinding will probably help you:
http://www.bay12forums.com/smf/index.php?topic=56041.0If you trust my reading skills, it can be summed up as such: Make as many direct pathways between the various parts of your fortress, make proper traffic designations, forbid everything that's not reachable, and designate proper burrows if you should go for a multi-fortress approach.
Now go forth and do SCIENCE, so I don't have to!