If I delete imovie, will all my files stored in "imovie library" also be deleted?

It shouldn't delete those files. However, if you attempt to load an older iMovie than created those documents, they may remain unreadable until you get the version up that did create them. If you aren't able to upgrade now, just upgrade the iMovie to a later version when you can. Catalina requires 64 bit compatible versions of all applications.

You can partition and install an older OS on your Mac using Disk Utility.

Mid-2015 should have shipped with 10.11. What that means is your Catalina partition will be invisible to 10.11 if you leave it at 10.11. But you can always upgrade it to 10.14 and keep the old 32 bit compatibility.

command-option-shift-R allows you to reinstall the oldest operating system that shipped with your Mac.

Once done, follow these steps to upgrade it to 10.14:

https://discussions.apple.com/docs/DOC-13571

Be sure to give yourself enough room to migrate your old Movies folder over when you partition the drive. Your boot drives should never be more than 85% full.

Deleting iMovie shouldn't touch the iMovie Library.