Sewer line replacement

Due to tree root problems, I've gotten a few estimates for sewer line replacement from my house to the curb.
I have two questions. 1) if it turns out that my existing sewer line is 6" clay pipe, is there anything wrong with the contractor inserting 4" PVC pipe through existing clay pipe for some of the installation rather than removing the existing clay pipe? The reason for doing this would be to avoid having to tunnel under a staircase that ascends in a direction perpendicular to the pipe and which is located above the pipe. By inserting PVC pipe through existing clay pipe, this could avoid damaging the staircase or causing it to settle. Is there anything wrong with inserting the 4" PVC pipe through existing 6" clay pipe in a problem section rather than removing the clay pipe altogether? None of the clay pipe would be used, since the PVC pipe would simply be inserted through it to avoid a problem area. Is this acceptable?
2) if the existing sewer line is made of metal (cast iron?) and there are no joints in it between the house and the location where it extends beyond the staircase (11 feet from the house), would there be any problem with the contractor attaching a PVC pipe to this location (11 feet beyond the house) rather than starting at the house itself. The contractor seemed confident that if the existing pipe is made of metal (cast iron?) and is good shape, and has no joints, that he could simply attach a PVC pipe to it with a solid connection (I think he said a 4 band stainless steel) and seemed confident there would be no chance of root intrusion at that connection. Is it true that with the proper connection of a PVC pipe to a metal pipe that there is no risk of root intrusion, or should I insist that the PVC pipe's connection should begin at the house (and take the risk of potential damage to the steps by having them tunnel under them.) I think the contractor may have talked about using a 4 band stainless steel connector rather than a rubber connector. As much as I'd like to save the steps, avoiding root intrusion is more important than saving the steps. Should I insist that the PVC pipe begin at the house or would connecting to the existing pipe be as solid in terms avoiding any potential root intrusion?
Thanks.
Jeff
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