A little more background might help:
HQC is a hard science setting. Although things might seem odd except in those places where we out and out break things for story reasons (fairly few and far between) we've looked hard at the numbers and done the math and science.
The aliens need a servant race capable of working in the high gravity of Earth and Venus (they lost a lot fot heir tools on the way here-an even longer story)
Mars is as we see it today when they start, as is Earth, Luna and Mercury. Venus is very different. It is actually mostly oceans with lowlands of the landmases being very dry and releatively cool deserts (think the Atacama in Chile). The highlands were the land intersects with clouds are very wet cloud forests. We crunched the numbers and the high albedo of Venus would give a surface temperature slightly below that of Earth, if the atmosphere was almost entirely transparent to reradiated infrared radiation. The pressure is very close to earth normal as well.
The aliens want the first batch of servents in a few generations, so they don't have time to do the breeding.
Mars will have a thick atmosphere by the time anyone is playing unprotected on the surface and we gave Venus a hefty magnetic field, and Mars as well, to ward off much of the charged particle radiation, so the surface radiation isn't much higher than Earth normal. People can actually handle a lot more low level radiation than it might assume at first glance. Even near black sand deposits there aren't increased cancer rates. (
http://www.nti.org/learn/facilities/342/). High altitudes see far more radiation than sea level populations and don't see a higher cancer rate. Considering how much radiation a smoker ingests over time and the fact that they die at fairly low rates is a good indication of just how much radiation a human body can handle, if it isn't a massive acute dose.