I'm not sure if I will be able to answer all of this- but maybe some! I teach preschool special education. My classroom is set up as a "self-contained" room (all children have IEP's). We have a total of 5 preschool classrooms in our school-2 self contained, 3 integrated (6 children with IEP's/12 without).
My own philosophy is that all children should be with their peers in an integrated setting as much as possible. Because the children in my classroom often are unable to benifit from a classroom with 18 children, I provide my students with "reverse integration" (several typically developing peers visit our room daily to participate in activities with the children in my classroom). We also integrate during our outdoor time which is benificial for the children.
For the majority of children I believe integrated classrooms, with necessary supports are generally prefereble to pulling children out and providing services in a "self-contained" setting. However, there are always children who will need a smaller setting with more adult assistance and lessons based on their individual needs.
There is never a cut and dry answer- and I think looking at the child as an individual and providing the necessary supports in the least restrictive environment is always what we should aim for! Hope I answered at least some of your questions! and, of course, this is all just my own opinion!