I have never tried using the PLE300 system with the PLE400. However, I did find this article on the Cisco website that states "PLEK400 is only compatible wiht PLE400". Would recommend contacting Cisco support to verify. Here is the link I found: http://homekb.cisco.com/Cisco2/ukp.aspx?vw=1&docid=77aeb2cd657f4452b10ce5b0cffcb663_PLEK400_Frequently_Asked_Questions.xml&pid=80&respid=0&snid=6&dispid=0&cpage=search
I can't be entirely sure since I don't own a Ple300. However, in addition to the Plwk400, I also have a Netgear Powerline unit. The netgear works transparently with the Ple400 unit - which is one of two pieces in the Plwk400 set. My Ple400 connects to my router while the wireless-n piece is located elsewhere across the house. The Netgear unit happens to be in the same room as the wireless unit and is hardwired to my blue ray dvd player. These 3 units coexist seamlessly. That said, I was unable to find anything to convince me that the Ple300 would be incompatible.