Adding pictures to an exiting catalog

New photos (regardless of where they come from) go into the same catalog. Keywords and other metadata within the catalog are the ways to keep things separated and keep them identified. Automatically, as soon as you do the import, without you telling LrC to do this, the name of the camera/phone that took the photo is part of the metadata and so you can always search by that metadata to find photos from a particular source.

I divided them into several collections based on the date they were taken. 

Collections are a particularly poor tool for this. Better you should, at import, tell LrC to make folders by capture date, it will do this for you, rather than you creating collections and then making the photos members of the collection. Another way to do this which is far superior to collections is to use the filter bar, and then you can select photos by date, or by month, etc. this happens automatically for you without you having to create collections and then make the photos members of the collections.

Will these additional photos be easy to find or locate?

If you use the tools in LrC, like keywords and other metadata, yes. If you don't use the tools or don't use them effectively, then probably not.