yes the role is determined by his skills....and changes as you train a player
so if you get a 5'11 c and start training his outside skills he will become a guard in some time....
the reason why there are no 7'1 PGs or 5'11 Cs is that height is a really important thing on training... a tall player trains his outside skills slower than a 5' 11 for example.... and the same for Cs... a small Center will train his inside skills slower than a 7' one....
I hope I answered you questions :)