Yeah, I've heard differing viewpoints on teflon, but, what I've tended to find out from other plumbing projects is just not to over tighten it past hand tight. I used teflon on my plumbing and just made sure it wasn't overly tight.
If the T is 3/4 vs 1", I'm reducing my return a lot. If I reduce it after the T, there's no reduction on my return pump's capacity. (it outputs at 1"). Because you are eventually reducing the 1 inch line to a 3/4 inch line you will still have a reduction in the flow rate even if you reduce the line to 3/4 after the TEE. The flow will just be reduced at the tee instead of the outlet.
What do you mean similar types of PVC for the return? You mean 40 vs 80? Shouldn't matter...Because the diameters of sch 40 and sch 80 are different you would have to use a coupler to go from one to the other.