Yes, it is done all the time
Downstairs? First floor with a basement or in the basement? If first floor with access in the basement or crawlspace, no real problem. PVC, just cut the pipe and install a T. Cast iron, you can use a Saddle Clamp made for this. Looks like a PVC T split length wise and held to the cast iron with two U bolts. If adding a bath in the basement, you will have to bust out the floor, add the drain and replace the cement.
No less then $6,400 plus tax plus filing fee and inspection fee's patching and back filling is normally an extra
3/4 inch would be too small for anything but just liquids. As long as there is a trap under the sink, there shouldn't be a health hazzard. You will have to be a little creative in connecting the sink drain to 3/4 but possible.
Where? What is your question?
By cutting in a cast iron Wye so the flow enters the sewer line (soil pipe) at a right angle
With a compression mission ring
The shower may be leaking from: 1) A failure of the drain and associated piping itself (e.g., corrosion of the cast iron floor drain), 2) A failure of the shower pan, which is an ostensibly waterproof liner (may be metal or synthetic polymer) embedded in the tile floor (or comprising the plastic floor) which is mechanically connected to the drain; or 3) Leakage of water in the shower's surrounding area (walls, non-pan floor area, etc.) which is accumulating and dripping near the drain.
More information needed. Are we talking fixtures, supply piping, drain piping? Pipes in a slab floor, crawl space, basement? What kind of pipe galvanized, brass, copper, cast iron, pvc, polybutylene, pex, abs? Other than that simply said rip the old pipes out and put new ones in. In your case the lack of information implies that you have not much of an idea in what is entailed and I would recommend calling a licensed plumber.
1- What kind of pipe Gas, water ,waste ? Water2- What type of material black steel, copper, brass, cast iron PVC, ABS, galvanized ? Cast iron3- Joints Soldered, welded ,threaded, glued, soldered, brazed, compression ,flaire , Caulked, No Hub , MJ? don't know
The cast of Basement - 1998 includes: Sally Venard
The cast of From the Basement - 2012 includes: Moby as himself
The one going out of the wall or floor goes to the septic. The one going up is the vent that should go out the roof.