You are correct, the 10 would fall out of the Gp100 holster upside down after it has been worn a while.  I'm sure the Gp100 will fall out to after a while too. 
I am mainly trying to be able to build holsters for the 4" K frame.  The 686 I am not to worried about because I can buy a bluegun for it or use the GP100.  With the K frame it is harder to get the variations.  The 4" K frame bluegun will cover the 10/64,13/65 with a heavy barrel, but then some of the 10/64 have pencil barrels.  The 15/67 has adjustable sights and the 19/66 has adjustable sights with a half underlug so there are some big differences on a molded and pressed holster. 
I would like a dummy gun for each model because like you I make holsters for the specific gun.  I am trying to figure out if I am being to specific.  My main problem is I have not been able to find a 15 dummy gun or a 19 dummy gun.