What are the minimum hours required for private instrument fixed-wing plus commercial helicopter?
The 150 hours is flight time which is not category or class specific so hours in both airplanes and helicopters would count towards the 150 hours.  You are missing an instrument helicopter rating.  It is in addition to the instrument airplane rating.

Below I try to sum up the minimum hours required for each certificate and rating if obtained under Part 61.  Part 141 schools require different hours.  You actual experience will vary considerably.  I would first get a private pilot airplane and helicopter certificate and then work on the instrument ratings concurrently.  Afterwards, work towards the commercial helicopter rating.




To get a private ASEL certificate:  You need a total of 40 hours
flying time to that meets 61.109 experience.
To get an private helicopter certificate addon:  An additional 30
hours of training is required.  You will already have more than 40
hours total time.  Total time at this point would be 70 hours minimum plus the flight time for the practical tests.
To get an instrument rating for both airplane and helicopter: You will need 50 hours of PIC cross country time plus 40 hours of simulated or instrument time.  The 3 hrs simulated time required by the PP ASEL should count towards that 40 hours.  You must have at least 15 hours each with a CFII in both airplane and helicopter (total 30) and if they are all cross country flights that will help your cause.  You already started with a minimum of 8 XC PIC hours from your private certificates.  The remaining 12 hours along with 7 hours of simulated time can be accumulated in either airplane or helicopter.  Total time up to this point should be 120 hours plus the flight time for the instrument rating practical tests.
To get a commercial helicopter certificate:  A total of 150 hours total time (all the above counts) and 50 hours of helicopter time.  (You should already have 45).  35 hours of PIC helicopter time (Already have 20) to include 20 hours of training on commercial maneuvers, 10 hours solo and 10 hours of helicopter cross country time.  The way I see it, is the commercial will add an additional 30 hours to your total time giving you exactly 150 hours.