Just some notes on astrophysics:
- If you break up one star into a binary, the energy output (after it settles from the action) will be dramatically lower. You'd be turning one yellow star into two dim red ones, or collecting massive amounts of hydrogen from somewhere.
- Planets can easily be ice in a bright/multistar system, they'll just be farther away.
- Orbits in multi-star systems will generally be unstable. Really close to one star, or really far from the group of stars, or locked into a resonance with a binary's orbital periods would work though. (Planets at the lagrange points of a pair of cool stars?)
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As an extention to the breathing thing, perhaps a couple of classes of breathability:
- Optimal
- Filter/concentrator-Assisted (passive, gas-mask style)
- Isolated-assist (active, scuba tank type stuff)
- Fully Domed ("space suits" for outdoor activity)
- Deadly (Just too nasty, people in domes still die off regularily)
Some sort of piecewise curve for productivity/reproduction/death rate/pop maintenance costs
You'd need to be in the first two categories to have no major penalties to population limits on that part of the planet, the first three to avoid major work penalties, etc.