This is untrue.
Changing population from worker to specialist will deduct 0.75 from population unhappiness and add 0.75 to specialist unhappiness.
Weird because game files say 1.34.
So, once you've got the energy boom going, how many buildings do you rush buy in each city? By my count, you need 15 buildings per city plus 3 trade routes just to take care of the unhappiness from the city itself with Industry, much less the population. Rush buying health buildings will help, but only up until around pop 8. Though I suppose if you're spamming generators everywhere your cities might not be growing past that much. Sprinkle in a bit of baseline health and you'll probably stay in the green, but I don't see how you're reaching that 20+ breakpoint.
Basically, with my Supremacy strategy I open Prosperity to jump start my expansion and secure lots of health bonuses before moving into Knowledge and getting Learning Centers with Academy spam to drive up science. I don't think Industry substitutes for Prosperity well in that strategy.
As far as Workers go, if I'm using a strategy based around basic improvements (i.e. farms and generators), I find 2 workers per city to be sufficient, though a few extra when magrails become available doesn't hurt. If it's a strategy based around more advanced improvements (i.e. biowells, academies, and/or terrascapes), then three workers per city is much better as you push out those new improvements, after which you can cut back on workers as things fall into place (PAC can obviously do more with fewer, and Master Control can have a big impact on how far you can stretch your workers).
Exact buy rate is influenced by expansion rate, war rate, and exact degree of boom. and yes pop stays low, 10-12 tops, you don't need more with that kind of city density. I linked a screenshot of my last attempt, health is lower than it should be as that slav city is costing me loads, (just grabbed it and it has only a crytonusery for health), and the war force me to re-found a city whilst diverting a lot of money to the war so i have 3 cities with some or all of the health improvements missing and only a few buildings and upkesh is missing trade routes.
Linky.
This is a Harmony game, but i don't see a supremacy game adding much extra in the way of issues. Simply drop alien sciences et all and go downwards instead of across. I do have saves if you want a look see, though as i noted i'm still perfecting this strategy.
Anyway gtg shopping, back later.