In knowledge articles, you can’t select neither software nor hardware items from the Related Catalog Items related list. The reference qualifier is getting only Record Producers and Catalog Items.
Out Of the Box, the dictionary entry for sc_cat_item on kb_2_sc table is configured to only show items whose class is Catalog Item or Record Producer.
To configure it to show software or hardware items, simply add conditions to show items whose class is Hardware Catalog or Software Catalog.
It should look like this when done:
As always, test thoroughly in a sub-production environment before implementing in production.