What is the safest way to remove an old cast iron soil stack. Is an
angle grinder suitable to cut the pipe into smaller sections?

This needs to be done as the separate w/c and existing bathroom are
being converted into a shower room with w/c and a smaller family
bathroom again with w/c. All the other wastes from newly installed
showers, baths and basins will then connect to, via appropriate sized
bosses, a new plastic soil pipe.

Thanks

Ed