Hi @Rimi_Marke,
The answer to your questions can be found in this community post by Kristy, but pasting here for convenience!
Yes, you can use Knowledge Tags with your custom entity types! Custom entity types will show up in the list when you go to select which entities you want to add Knowledge Tags for.
Keep in mind that because custom entity types don’t have predetermined Knowledge Graph fields, you will need to manually configure the schema.org markup on these pages to ensure that your pages are eligible for rich search results.
Check out this unit on customizing schema.org markup and this help article on customizing schema.org markup using Knowledge Tags to see how.
Thanks!
Jamie