My approach looks at this from the angle of providing buildings per citizen, rather than limiting citizens per building. Buildings now have variable cost, depending on how many citizens it will provide for. Imagine you are building a factory, but only have 2 citizens in the city. It makes no sense to build a facility that provides for 10 citiznes(assumption for current factory), so you build one that is only 1/5 the cost. I'll explain how bonuses and demographics work, but first is cost.
Cost:
There are several stages to cost, but two types of stages. One is fixed, the other is variable. Fixed costs represent the basic infrastructure needed for that level and size of complex. It only needs to be paid once in order to build to the next level of variable facility. Variable costs are those based on how many citizens you are providing for. You get to choose the exact number. As a side note, you can add on to facilities anytime you want.
Here is an example, made up numbers, for the stages for lets say a factory. THese names would not necessarily appear, it would all say factory. They are their for discussion purposes.
Rudimentary Infrastructure(20 shields, Fixed)
Factories(1-4 citizens, 10 shield per new section/citizen, Variable)
Incoporated Stucture(30 shields, Fixed)
Large Factories(5-12 citizens, 15 shiled per new section/citizen, Variable)
Integrated Facilities(40 Shields, Fixed)
Manufacturing Complexes(13-25 citizens, 20 shields per new section/citizen, Variable)
Jumbo Inudstiral Operations(50 shields, Fixed)
Super Manufacturing Facilities(26+ citizens, 25 shields per new section/citizen, Variable)
Effects of Buildings:
Lets say you have a size 6 city with a size 3 factory. that means only the top 3 sheild producing squares get the benefit of the factory(+50% production). This might be useful for a city with only 2 or 3 high shield squares, since the cost would be 40-50 shields instead of 160.
Demographics
The important part fo demographics is they are who recieve the products of the buildings, not produce them. Remember, each building allows for a certain number of citizens to utilize the bonus, but the products go to that smae number of the 'richest' citizens. Basically the first citizen on the left of the screen gets each thing that is avalaibel first. Some people may have one service but there was nto enough of another to enjoy its use. I'm not sure the ultimate consequences of this division of welath, but it could be very useful.