Imoprting the photos into one catalog does not prevent you from importing photos into a different catalog later ... however ...
I strongly advise you to use a single catalog for ALL of your photos, and I strongly advise you to use keywords or other metadata to organize your photos. Do not use Catalogs as an organizational container, unless you have photos whose subject matter is COMPLETELY non-overlapping, like personal and business. Otherwise, use keywords and/or other metadata to organize and segregate and search.
Why? There are no search tools in Lightroom that operate across multiple catalogs, so if you have to search for a photo, and they're all in a single catalog, you can use Lightroom's search tools. If you have the photos scattered across multiple catalogs, you cannot use Lightroom to do the search.You are defeating the purpose and defeating the capability of Lightroom.
Furthermore, with multiple catalogs, you would have to constantly switch catalogs as need, and you run the risk (as you ahve already done) of importing photos into the wrong catalog. None of these happen if you use a single catalog.