As you already hold a commercial pilot certificate, there is no need to obtain private pilot privileges in an airplane...you can add on at the commercial level. However, remember that you're learning an entirely different kind of flying. Before you jump into multi engine flying, you should get some good single engine airplane experience under your belt. If you're just looking to add the rating to your certificate, it's not a big deal...but if you want to use that rating you'll need some solid fixed wing experience.
For this reason, many will obtain their fixed wing certification at the private level first, in order to begin logging their fixed wing time as PIC. Its not necessary, but it's a choice that many folks make to enable them to log more PIC during their training.