corruption is stackable - the way the corruption model works, each building that has reduces corruption lowers the corruption and also lowers the threshold.
I am not sure why you have a hospital reduce human pollution... since it is the thing that makes human pollution possible in the first place. (except for shakespeare's, of course)
I suspect that changes like that will make for a much more powerful AI, since they don't make enough workers as is (so less pollution helps them) and they build temples/caths reflexively, so they will have very little corruption.
I don't know about pollution, though. The way that I understand pollution reducing improvements to work, they basically set pollution chances to 1% for each type. (ie, a size 20 city has an 8% chance of human pollution - with mass transit, that turns into 1%), so an additional improvment would probably do nothing.