Only to add that any one image that is common to more than one of your small catalogs will obligate you to be aware that during the Import From Catalog step, you'll be over-writing prior "versions" of that file when you're making your serial importations. You have the option of saving the "prior versions" and making new virtual copies upon subsequent importation.
The suggestion that you first start by being sure that your segmental catalogs are in order, and can find all image assets, is correct as I see it. Only then does it make sense to coalesce your small catalogs into a master through the Import From Catalog process.
I work the way you're describing: I have individual assignment-based catalogs in which I do all my Library and Develop work, and subsequently import that content into one large Master Catalog. Others decry this approach, meaning they feel the small individual catalogs idea is poor.