This whole situation bore further watching, especially since Nodoka said

she considered Ranko to be her niece instead of actually being her niece. 
If they were not related by family, then perhaps it was time for her to
try something to encourage Ranko and Ranma.<<

One would think that Urd would be trying to solve Ranma's love problems than trying to add to them. Ranma already has 4 love interests. Although Urd's interference has added to Keiichi's problems in the past, she has never intentionally and knowingly caused problems for anyone. Getting Ranma and Ranko together would be totally unlike her, instead she would be trying to get Ranma with one of his love interests.
But the idea of Ranma being affected by the combo love potion you created is intriguing, perhaps you could find another way to use it?

I had some more things to say but I don't your fic with me at the moment. Later.