@ChipM 

The problem with using the Archer C7 and or C8 with the Deco P9 is that you would have essentially 3 seperate network  created.  The Deco Mesh is not designed to work in conjuction with the Archer C7 or C8 traditional routers.  While you can put the C7/C8 in AP mode you will still have their wireless creating seperate APs for devices.  Even if you use the same name and used Powerline adapters to connect the Archer C7/C8 in seperate locations and did not have the wireless of those devices overlapping the Decos you would still see connection issues as your client devices tried to decided what AP to use. 

The Best bet would be to get a few more Deco units and add them to your Deco network to expand the coverage under 1 single AP network.  You can setup 10 total nodes which includes the node connect to your fiber drop.  The Decos also do not have to be the P9 either. they can be any of the Decos on the market. I would advoid the W2400 though.  That is only because its ethernet ports are fast 10/100, so if you connected something dirrectly to them you would be limited to 100 Mbps.  All of the other Deco feature gigabit ports.