What determines if a nation is a valid rival?

I am playing as Ayutthaya and have selected Kutai as one of my rivals. However, I would like to replace them by Malacca (who used to be my ally) in order to get cheaper conquest of their provinces in terms of diplomatic power.

However, it seems that removing a rival comes with a steep cost of 100 diplomatic power which I would like to avoid paying. Since Kutai is quite weak, I think they might be close to being eclipsed. What are the conditions a rival must satisfy to be eclipsed, and similarly what are the conditions a nation must satisfy to qualify as a valid rival?