What I read in a book called " The Mongols" Shah dynesty is come from Iran ( then persia) when it came under attack during the rise of the mongols led by genghis khan. The persian ruler shah then fled to India and set up Mogul empire there and remnants of their relatives entred Nepal.

Politically experienced shah then conquered the western part of nepal which was then ruled by Magars.They gradually integrated themselves into humble magars and finally stuck a knife into their heart politically. According to nepalese history we all know that Drabya shah won the race and became the king of gorkha but I personally believe that the fact is not told. If you read the book called " The hidden facts in nepalese politics" you can find some facts what happened then.

It is widely believed that the Nara bhupal shah ( father of prithivi narayan shah) had  magarni mistress and prithivi narayan shah was born to her.

summery: Shah and Rana ( chetri) are modern iranian family names and I personally dont believe that they are decendents of  magars.