New here so I am working on my own fanfic, but i was curious if anyone
could come up with a reason why Cologne would train anyone other than
Ranma or Ryouga, say any of the girls.

	Ok Cologne trained Ranma because A, she approves of him marrying Shampoo 
and wants her granddaughter to marry a very capable martial artist.  B, As a 
sort of carrot, Martial Arts is Ranma's life and by training him she's showing 
him how much his martial art skill will benefit by marying Shampoo.
	As for Ryouga the only time she trained him was so that she could A, 
reinforce Ranma's understanding of her skill as a teacher, and B, to try to make 
Akane a prize that Ryouga would win leaving Shampoo Ranma's only sutor.
	As for her training someone else, I'd say that she would if she could 
arange it so Shampoo would have a better chance at Ranma because of the 
training, or possibly if something happened which make her feel indebted to a 
potential student.