You are correct. You're not skipping anything, however. There is one pilot certificate. Not a private and a commercial. These are privileges that may be applied to ratings on your certiicate or to the certificate in general. However, if you're a commercial pilot, you're a commercial pilot, and you hold a commercial pilot certificate. If you add a helicopter category rating on to that certificate, you can add it with private privileges, or commercial privileges. That's up to you.
There exists no requirement to obtain private privileges when adding on rating; you can do it at a commercial level first.
As for the cost, avbug was not wrong.
A Robinson or a Schweitzer today will cost you about one hundred eighty an hour to two thirty an hour. Costs have gone up.
A typical private pilot add on will run about six grand, assuming you can knock it out in 40 hours. To do it as a commercial, if you can do it in the minimum time of 40 hours, about nine grand. Add an instrument rating for four thousand. Add on a flight instructor for six thousand and instrument flight instructor for another two, and you've hopped up another six or seven grand.
Now add in additional costs such as examiner fees, hotel and meals if you're going somewhere to start the training, and then of course additional flying if you can't get it done in the bare minimum number of hours (a lot of folks don't...not not in fixed wing, or helicopters), and the cost gets there very quickly.
The Robinson is used for more instruction than any other type; sensibly, if you're looking for a civillian instructing job, you're more likely to find work in a Robinson than any other type. To get enough flight experience to qualify to teach in a Robbie under the SFAR, you'll need to boost up to two hundred hours, and that is going to run you right up to nearly fifty grand by the time you're done.
Having said that, most helicopter places are only going to hire helicopter instructors who took their training there. Or more accurately, there are enough students coming through that want the job that they're unlikely to turn to outside help. Not all, but many. With that in mind, your most likely training mount will also be the one you instruct in...train in Schweitzer's and instruct in them...but at the same time you'll be unable to go somewhere else to instruct in a robinson if that need should arise. Or rent one, for that matter.
As far as Civillian helicopter pilots, you're seeing more and more of them in the workplace today. A lot more.