Your cats all have different skills and levels (see the Skills section of this FAQ).
Which abilities you give to which cat and how you cross them with each other to get even stronger cats is up to you. You need to find a good mix of your cat crowd and their abilities. Many weak cats will help you less than a few well-trained cats.
Always send the strong cats to the strongest enemy cat or the biggest pile of fish.
Teach your new cats different skills so that later when you breed them, you create a cat that has learned as many skills as possible!