Metadata source: avoiding merging of the results

For a few days I'm working on a metadata downloading plugin with which I have a problem:
how to avoid merging results by calibre, when there is no isbn? Unfortunately the site is an ebook source itself and publishes them with seperate isbn for each format (usually at least three). I've checked a lot of thinks, including calibre code and 2 questions remain:

Is it possible to find out what is the format of the book at the level of a plugin?

Is there other way to force calibre not to merge the search results instead of adding isbn?