The only type of connector for plastic soil pipe and cast iron is a rubber adaptor. Go to our bathroom suite installation project and scroll to the tools section to see (and/or buy) one. The cast iron is not suitable for cutting into and fitting a new toilet and you will need to either replace this pipe in plastic ( follow this link for pipe http://www.awin1.com/pclick.php?p=12530 ... 071&m=1228 ) which makes the job easy. If you can find a suitable place to cut through the cast iron pipe and connect the plastic pipe make sure you clamp each section, either side of the rubber connector, to the wall fully.
Hi. Yes you have the theory. The new plastic pipe, called a soil and vent pipe must now follow the exact route that the old cast pipe too, to the outside of the building and into the manhole where all the foul waste will go to a sewer. At the point you need the new toilet you will need to insert a T junction or a Y junction, whichever you can get to fit. The open branch of the junction then goes to the toilet via probably a bend and a pan connector. Links to all of these are below. The pan connector slots into the back of the WC.
My Mrs wanted the toilet turned through 90 deg, the angle is quite tight and at the moment I've got a nasty space consuming S shape on it at the moment as temp measure. Rather than take down the whole stack above (about 10 foot of cast iron 30/40 feet up) I'm hoping to go right angle from the back of the toilet into a new exit point, seal up the old one and use the multiboss to fit a new [plastic pipe into the old cast iron one. Yeah, it will look a bit nasty - but its at the back of the hosue where it can't be seen by anyone and because of some seriously tight access it's a lot easier than trying to bring down cast iron. (i've got just enough room to get a ladder up)