You can take the written tests in whatever order you like, ATPL first, PPL last if you want (they're valid for 2 years I think). To be able to do the CFI check ride you need an unrestricted FAA CPL (basically an FAA CPL with an FAA instrument rating). You can do all the training at once, I know a couple of people who did the CPL and CFI training at the same time, I'd keep the IR seperate though. IR first, then CPL and CFI together would be my advice.