Designing Kitchens and Bathrooms - Tying a shower train into a toilet drain

I have an 1/4 bath (sink and toilet) and I have started installing a stand up shower unit. I have removed the sink, toilet and the floor that is on top of the sub floor. They had to build up the floor 1.5" because the sewer line that goes to the toilet is cast iron and sits partially above the subfloor so they had to cut a hole in the subfloor to accomodate about 1/4 of the diameter of the cast iron pipe that T's off the main sewer line. Anyway, the cast iron pipe that T's off the main line is aproximately 8 to 10" and flanged. There is then a pipe that goes into the flag and from there makes a gradual sweep up to the toilet. That pipe is about 10 to 11" long and I think it'a aluminum because a magnet will not stick to it and when I removed the toilet there was a rim that allowed it to be bolted witht he wax ring and toilet gasket and I had to bend that part to the gasket off and it was pretty easy. I was able to fold that rim up with a pair of plyers. Although, no one I have talked to believes that I would have an aluminum pipe - it has not rusted ither. My thought was to cut this aluminum(?) pipe back as far as I could and install a rubber boot. I would then attach a 4"/2" PVC Y. I would have to install a 45 up to the toilet and then take the 2" across the floor and connect up with the shower drain. However, I have been told by my brother that if I do this every time the shower drains the toilet will bubble. That doesn't bother me too much as long as it's not bubbling all over and causing a mess. Is this true and is there a chance that the toilet would back flow to the shower? I can't take the shower drain to the sink drain because that ties into the cast iron 12 to 14" off the floor - so it won't slope downward to drain properly. I really do not want to do any cutting on the cast iron. Will this work or do you have any suggestions?

You didn't mention a vent for the new shower. If you do what you are talking about, everytime you flush the toilet, the trap in the shower will be sucked out and allow sewer gas into the house. This is also a code violation and if you ever sell the house you will be responsible for re-doing it or at least disclosing it. Good luck.

There is a vent for the sewer line - you can feel cold air coming in through the sink drain since I have the sink disconnected. You didn't answer my question on whether or not this could be done this way. So now that you know I have a vent would you recommend this type of repair?

Okay, you have a vent on the sink. You ALSO need a seperate vent for the shower. The bubbling your brother was referring to is because the toilet will be sucking air from the shower line everytime it is flushed if you do not have a seperate vent. I would not recommend this type of repair.