I really searched for it but I just can't find a link on KSP-Wiki or somewhere else that explains how Stock Inventory works. I thought in stock KSP the only use of the Inventory is to place these experiments. But the PakRat stuff doesn't fit into the Inventory of a Kerbal to place it so I thought that there was a different mechanic on how it works that was specific to your mod. But it seems now, that I just don't fully understand the Inventory yet.
Sorry for that. If you know where I could find a good source about the Stock Inventory and how it works that would be super nice.