to answer briefly...
Q1: is it possible to throw the STi turbo on an RS. Sure, it's possible. but you're oversimplifying. it's going to take a lot more than just a turbo and IC to make it work. Think bigger injectors, a lot of new plumbing, new oil lines, engine management, hopefully a stronger block with upgraded internals, etc. etc. etc.
Q2: is there a way to get an EJ20 without a turbo and throw the STi turbo on that? Yes, there is. But again, it's not that simple.
Remember, anything's possible with the correct planning, designing, and most importantly, a big fat wallet. Good luck searching.