We do not have a record of sale for the ink set you bought so you may have purchased this from one of our EU Dealers. So I do not know if you bought a K7 system for matte printing only - or a K7 system for matte and glossy printing but only wish to install the matte system.
You should refer to the installation of the inks on the installation page: http://www.piezography.com/PiezoPress/technical-support/epson-pro-7800-9800-installation/
In the case that you bought a traditional K7 matte glossy system - and you want to print only matte, you will NOT install the PK or the GO.
In the case that you bought a Piezography2 system for simultaneous matte and glossy printing - you would not have shade 7 and in its place you install the PK. But this does not seem to apply to you. You say you bought a K7 system for matte and glossy printing. This indicates you have shades 2-7, MK, PK, and GO.
Because I can not see what you purchased I will assume that you have bought K7 inks for matte and glossy - but that you are referring to the Piezography2 K6 system which does not use Shade 7. And therein lies the confusion.
In this case the MK goes in the MK cartridge and the PK goes into the PK cartridge. Both of these are used in the black ink slot of your printer. In your case, you can leave the PK in the box and just choose not to make glossy prints. You fill only the MK cartridge and install it into the black cart position of the 9800.
You should NOT install the PK nor the MK into the slot # 5 because that is the slot you must install Piezography Shade #7 ink. Shade 7 goes into the Yellow position slot.
In your case, you will not install GO into the LLK cartridge because you do not intend to print glossy.... The LLK cartridge will not be used in any matte K7 printing - and we recommend that you use PiezoFlush in this position to keep the print head happy. You can choose to do nothing and keep the Epson LLK cartridge in place. You must install something into this position or the printer will not print. The only time this position will print is when you do nozzle checks.... But this is the gray area for you because you bought a K7 system for matte and glossy but are not going to print glossy. If you bought a K7 system for matte only printing, it would have included a bottle of PiezoFlush for the LLK position.
I hope that this answers your question.