First, you should be referring to this page for ink placement. Be sure to choose the chart for R1900/R2000/P400.
None of the K7/K6 inksets are only for PiezoDN digital negatives — they are also for regular Piezography printing on paper. You don’t see anything specifically for Selenium because all the K7/K6 ink options use the same positions. P2 is a variation of the K6 setup that puts HD Photo Black in the otherwise unused yellow channel. This allows both black inks to be available without having to do any switching. As you can see in the chart, the only difference between K7, K6, and P2 setups is what goes in the yellow position.
You mentioned that your ink shades are numbered 2-5. I hope that is a typo and you meant 2-6, otherwise you are missing shade 6. Also, please let us know which black inks you have.
Install the Epson driver for your P400 if you haven’t already. You will need this only for access to the Printer Utility in order to perform head cleanings, alignments, etc. No not accept any firmware updates.
Install QTR and Print-Tool on your computer if you haven’t already. You will probably notice that QTR does not have a profile folder for the P400, but don’t worry - you do not need to install QTR profiles.
For printing negatives with PiezoDN, you will run the installation command from the folder Piezography>Curves>P400-PiezoDN-P2. You will notice that there are 2 installation commands. Use the one named InstallP400-PiezoDNP2=R2000.command.
For making Piezography prints on paper, you will run the installation command from the folder Piezography>Curves-HD>P400-P2>InstallP400-HDP2=R2000.command.
You may have come across a suggestion to install the Epson inks first in order to check that the printer is working right. It is recommended as a safeguard for new printers under warranty to avoid the possibility of Epson refusing to honor the warranty due to the use of non-Epson inks. I recommend against this in most cases based on personal experience. I have never bothered with it on any new printer I have used for Piezography and have no regrets. (I think there is one model, maybe the P600, that does require the installation of Epson ink cartridges to initialize a new printer, but I’m not sure about that.)
Hopefully this will get you started and Walker will correct any errors I may have made.