cast-iron

I thought I had fixed the issue by adding an additional air intake to the line but the other day the tenant drained the washer and then water was pouring out of the sink/drain, I'm wondering if it's because there may be a clog where the new pvc wastlines from the kitchen, meet an existing 2"...

I have an old cast iron clean out that I would like to replace. It serves one bathroom and vanity. I was wondering if I will need to support the upper portion of the remaining pipe both during and after replacement. Is the PVC section going, along with the new PVC clean-out, going to be strong...