They aren't there with the intention of giving them a permenent home even if they are there a long time. Being healthy is happy for a fish and just because they appear healthy on the outside dosen't make it so inside. A fish could have failing kidneys due to high nitrate and you would never know it.
The only way to know water is pristine is to test it.
Aggresive fish injure and kill eatch other. They stress eatchother, weaken immune systems etc. There is nothing wrong with having aggresive fish, most of mine are. The difference is that mine are properly stocked and aren't aggresive toward eatchother. If aggresion becomes a problem I fix the problem.
That is news to me but I wonder. If you want something to look at why not get get fish that stay small? You won't ever see your fish at full size so what is the point? There are plenty of small aggresive fish out there
Honestly I cannot remember anymore, this is an old thread in my book and I can't even remember what your setup is like. Perhaps I'll re-read it but I probably won't bother.