As far as I am aware, there is no research so the best you might hope for is anecdotal evidence from a few instructors.
I'm not sure "hours to solo" proves anything. It can depend, of course, on the student's ability but also the instructor philosophy. Some students go solo early for the fact they've gone solo; others, much later. It doesn't matter.
For what it's worth, I went solo after about 40 hours and got my licence in 67. Some go after 10 but still get their licence after 60/70 hours.
With respect to your second question, if all you want to do is fly a helicopter, I don't see any point in getting fixed wing first. I didn't; in fact, I've never flown in anything smaller than a 737 and then I'm a passenger!! 15 minutes glider time is all I have!
It depends on what you want to do with your licence and whether that licence is PPL or CPL.
Cheers
Whirls