Juugo's KKG abilities comes from ryuchidou training in senjutsu. Juugo even confirmed himslef that the curse seal power is just another name for the original. Sennin power that allows the user to transform into a sage.
How is Juugo not a Ryuchidou snake or dragon sage?
I think I found where you are getting that info from. You are reading wikipedia stuff aren't you? Here is what kabuto thinks when you compare the curse seal to sage mode. http://www.mangahit.com/naruto/579/11
Juugo was not trained at there. His body actually has a some what natural ability to gather it but the problem is unlike real sages which transform properly into real sages, Juugo and his clan transform into abominations that can't use the real power of sage mode. They can only optimize their existing body unlike real sages which can upgrade. In essense the best way to compare this is Dragonball Z super saiyan 2. When trunks and goku first tried to go super saiyan 2 for the first time they did it poorly which resulted in them transforming into an oversized brute that was just a pumped up version of super saiyan. When gohan on the other hand transformed into super saiyan 2 he did it properly with a true transformation. Juugo may be able to gather sage energy but compared to the other he is not in any way a real sage. If sasuke were to try and use juugo's cells he would just have the same effect. You need the real training at the real place otherwise the version of sage mode you end up with is just a piss poor copy.