Some people keep loads of large media files, so they need more storage. If you don't need that much storage then, yes, use the SSD for both Windows and your data files. As for "copy everything to new the SSD", if you mean to backup the existing drive and restore to the new SSD or do a clone, I would recommend a different approach. Your computer is five years old, so I think a clean install would be beneficial. That's what I would do and what I have done on many laptops.  Make a full disk backup of the current drive to an external drive. Remove the HD and replace it with the SSD. While you're in there, it would be a good idea to clean the fan and heat fins, replace the thermal paste and the CMOS battery. That may be more work than you wanted, so I'd understand if you want to stop at installing the new SSD. Using Microsoft's Media Creation tool, download a Windows installation ISO to use with something like Rufus to create a bootable flash drive. Or, let the Media Creation tool create the flash drive directly. Boot from the flash drive and install windows from scratch. After setting up Windows, you'll have to install your applications, so be sure to save any licence keys before removing the HD.
I have an old Dell 7559 that is a pretty good computer still.  I just bought a 1TB SSD hard drive ( Samsung SSD 860 EVO M.2 2280 1TB 1.0TB SATA III) I plan on installing to speed her up a little.  I'm now realizing the old HD will stay installed which is a 1TB as well.   I am thinking it would be best to copy everything to new the SSD and just run completely off of that?  Data and Windows.  I notice a lot of people are using the old HD for storing data and the new SSD for just Windows, that's what I see in the youtube videos.  Would it make sense to copy everything and then erase the old HD and maybe use that as a back-up image in case of SSD failure?  Any recommendations?  I'm just trying to figure this out before the parts come.  Thanks!
Dell Inspiron 7559 Upgrading hard drive, help please