Sharepoint - Is there a way to create classic pages on modern sites with Sharepoint Online?

From your description, it seems that you want to use both the modern pages and classic pages in SharePoint Online.

If yes, I think you can achieve that via he following steps.

a. Navigate to a library.

b. Click Return to classic SharePoint>Settings>Site contents>Site Pages>New to create a classic page.

c. If you click Settings>Add a page, you can still create a modern page.