Personally I believe you can be a mix of those, Ecto-Meso, Endo-Meso etc etc It depends on so many factors, and YES I believe you can alter them!
I was a super skinny growing up. the freaking wind could blow me over. But I've been training for almost 10 years now, now I weigh about 230 lbs. I've been as high as 265 lbs!