adding shower drain in slab

i am adding a shower to my bath. the center of the new drain location will be aprox. 3' away from the center of the toilet drain. would it be ok if i tie into the cast iron toilet drain? and do i need a vent for the shower or can i use the vent for the toilet? also how do i join the new pvc 2" shower drain to the 4"? cast iron drain.
thanks