Two old versions of Lightroom and two catalogs

[default0arbg9sgjaa7]

I have two old versions of Lightroom on my Mac (Adobe Lightroom CC Version 2015.5.5 and Lightroom Classic CC 8.2.1) and want to switch to the combo of LR and LRc. To make matters more complicated, both have different catalogs (15,000 photos in Lightroom 2015.5.5 and 1800 in Lightroom Classic CC). How did this happen? I bought a Sony A7Riii last year and discovered that my old version of Lightroom couldn't read the ARW files. And instead of spending time figuring out the issue on my own, at my work IT guy's suggestion, I just opened the other version of Lightroom that was on my (office) Mac. When I did, I found out that it not only opened my A7R files; it had many other cool/better features. So I just started using it.

Ironically, the impetus to change again was because last week I purchased.. wait for it... another Sony camera: an A9ii. Whose ARW files won't open in the version of Lighroom Classic that I have. In any case: I'm fully aware that I should be rolling with the latest versions. And I'm almost certain LRc will open those A9ii files. My question is: is it possible to import both of my catalogs into LRc (which can then by synched with LR)? And if so, how? I haven't opened LRC yet fyi. Or the new LR. So how do I prepare the ground? Or do I just save the files in (especially) my old Lighroom CC to be opened as needed in LRc (since some go back to 2013). Thanks for any help anyone has to offer. 

[ManiacJoe]

The newer versions of LR have requirements for the newer versions of the Mac OS. Check the system requirements for LR.

You certainly don't need the old versions of LR. Feel free to uninstall them.

Each time a new version of LR was installed, the old catalog should have been migrated to a new catalog format with a new name, often with a "-2" added to the catalog name.

Yes, you can merge the multiple catalogs.
First using the most recent version of LR you have, open each old catalog and migrate the data to the new format.
Then open the catalog you want to be the master catalog, then use the "import from catalog" command on each of the migrated catalogs.

Check the list here to see if your camera is yet supported:
https://helpx.adobe.com/camera-raw/kb/camera-raw-plug-supported-cameras.html

[default0arbg9sgjaa7]

Thanks. Much appreciated.