I had the same thing. I disabled all of the functionality (Wireless, DHCP, etc), changed the IP (from 192.168.1.1, as the Velop uses that IP as well) and changed the hostname on the EA9500 and set these up with the same SSID that I had on the EA9500.
The additional devices only require power - no extra Cat5e/Cat6 cables needed. Since the Velop doesn't have a bunch of ports (just 1), I kept the EA9500 to act as a switch.
Spreading out the Velop units DOES make the signal better with little to no impact on speed.