Well I'm new here, but I think I'll get to the point.
As far as Andalites not growing weaker, and if anything, stronger, I think this is caused not by natural selection, but by selective breeding.
Ax originally feels extreme hate towards vecols and other invalids or disabled people, not because of his character, but by what his society has taught/told him. Most likely, these people are shunned in an organized fashion to prevent the mingling of 'bad' DNA with 'good' DNA. I remember from one of the chronicles books, I think it was the Andalite Chronicles, but it may have been Hork-Bajir; the respective Andalite of the book performs the wish-flower ritual and says something along the lines of, "With the war going on, they may start letting some families have children." It was either Aldrea referring to her younger brother or Elfangor to Ax.
Either way, this demonstrates that the Andalite government has at least some form of when kids could be had, and possibly to who. It's also well known that the Andalites have pushed something of a strong sense of what could be compared to nationalism to their people. Initially, Ax is extremely steadfast in the beliefs of his people and is for a while is disbelieving they could do wrong. This is likely caused by propaganda, probably put into effect behind the scenes. I remember in the book where Ax meets the other Andalites for the first time (with Estrid... I think it's 38), the 'Apex Level Intelligence' or something like that, basically controlled the Andalite council. If it's anything like this at all - War and corruption while the people see nothing - the Andalites could be made to try to form the "Aryan Master Race" of sorts. In other words, "Your tail blade is small. You will not have Children." The propaganda fed and nationalistic Andalites would happily comply. This, coupled with very likely behind the scenes bio-work, could easily make them stronger with extra senses (Time, etc) than what would naturally occur.
As for humans, put simply, we're opportunists. If there's an easier way to do something, we do it. Why work fields when you can ride a tractor? As for adaption, we've had no choice. A small population in a temperate region with tail blades for keep predators at bay wouldn't have trouble. A relatively small mammal with no natural weapons, low strength, and low speed needs to be able to exploit every possible opportunity to survive.
