One more question: is possible to remove the second floor of the cage but still retain the upper mesh?
I am assuming you mean the upper mesh of the lower level? Yes, that is possible.
I will give you my view on breeding/adoption just so you can see the other side of it. Firstly, I definitely appreciate that you have looked into adoption, educated yourself, and weighed out the choices before deciding, and didn't just choose a breeder because they are "cute little babies." I don't think
anyone should be breeding rats
at all, whether that be in a crappy rat mill or a well respected and "good" breeder until there are
no unwanted, homeless rats available (or very very few). By adopting you are not supporting the rat mill/pet store trade (sadly someone else has likely already done that), you are supporting a local animal rescue (always a good thing!), and are helping a few rats in need at the same time. What could be better than that? Adopting doesn't just feel good, but it is doing good.
Yes you won't be getting the "pure genetic lines" (in quotes because most breeders claim this but cannot deliver), but does this really matter? The only reason this would matter is because you are going to breed yourself, which would not be ok.
As long as you get happy, healthy, well behaved ratties, who cares about the genes? Any good rescue out there will first quarantine or screen the rats for health and temperament for at least a few weeks, if not a month or two, before putting them up for adoption, and will guide you through the process of adoption to find the best rats for you. They will also accept the rats back if there are any issues with compatability, temperament, or health after you take them home. You have just as much as a guarantee for happy, well behaved rats as any breeder out there, if not, more.
Rats from breeders have just as much chance, if not more (since most breed indiscriminately) to have health problems later in life, like PT, mammary tumors, strokes, HED, etc, and to be given up by their owners (since most breeders don't screen buyers properly). So, assuming that most or all rats up for adoption in rescues originated from pet stores/rat mills is misguided. I would say its an even mix between breeder rats and pet store rats. And there are still LOTS of pretty colors and patterns in shelters and rescues.
It definitely should NOT be the norm to get rats from breeders. 99% of them are mini rat mills, and are doing all the things you listed (breeding back to back, removing litters early, not screening for health or temperament, inbreeding, etc) and only care about the money, and absolutely nothing about the rats themselves.
It would be amazing if rat mills were shut down forever, and pet stores were banned on selling live animals, but they aren't and won't be for the foreseeable future (as sucky as that is). We do not live in a perfect world, we live in a real world. In the meantime, you can help the problem by adopting homeless, needy ratties until that happens (crosses fingers that it will someday happen). Not adopting rats from shelters will not stop rats from being bought in pet stores, or being given up by their owners. By buying rats you may not be supporting rat mills, and may be supporting a reputable breeder, but you are still paying for more rats to be brought into this world unnecessarily when there are literally thousands out there already born needing homes.
You also have to remember: breeders (good or bad) do this for the money (as much as they say they don't). Rescues don't. Most if not all of them lose money every month by doing this, but still do anyway because it is the right and humane thing to do (and, yes, it does feel good, whats wrong with that?). Hundreds more rats a month would be put to sleep or fed to snakes if rescues didn't exist.
You have obviously figured out that this is a very pro-rescue forum. I think its safe to say that we will not judge if you choose to get rats from a breeder (if you have done your research, which you seem to have done), but we strongly encourage adopting rats already born than paying to have more brought into this world, no matter the source.
Again, just wanted to share my side of it.