Do I have to design my files in metric because I see that UCCNC is metric?

No, You can design in inches or metric.  If you are using Vectric programs, you can design in inches or metric because when you choose the post processor to create your file, the post processor will convert the G Code to metric.

For more information on which post processor to use, please see this FAQ: https://cncfaq.us/ufaqs/post-processor-use-vectric-programs-vcarve-cut-2d-cut-3d-photovcarve/