Thank you for your clarification on the Catholic view of souls. Would the term “separately-created souls” instead of “pre-existing, pre-created” be accurate?
You wouldn't want to imply that the soul was created "separately" --- it is at no time separate from the body of the newly-conceivced child. But it is, Catholics would say, "individually" created and infused as the body comes into being. In other words, you get your body from your parents, but your soul comes straight from God. Here's a Link to some Catechism references if you want to get it in context.