c6300 modem only for orbi AC3000

Yes, the existing C6300 will be compatible with the Orbi WiFi system.  Some things to note:

• The WiFi radios on the C6300 will need to be disabled so that they do not compete with the Orbi radios.
  My practice is to name each new WiFi router with exactly the same WiFi name (SSID) and password as the previous router.
  That way, (a) all of my WiFi devices will happily connect to the new WiFi system without needing to be reconfigured, and (b) should I decide to "go back" to the previous WiFi router, all I have to do is unplug the new one and activate the old one.
• Most of us on the forum recommend having only a single "router".  Orbi will work just fine when connected to the C6300, however there are a bunch of features that will NOT work when behind another router (OpenVPN, some Gaming apps, etc.)  Here's what Netgear says about "Double NAT": https://kb.netgear.com/30186/What-is-Double-NAT 
• There is also a potential issue when devices are wired directly to the C6300 and the Orbi is in its default "router" mode.  There will be issues trying to access to/from the devices connected to the Orbi and the devices connected to the C6300.  This is another reason to either (a) put the C6300 into "bridge" or "passthrough" mode and have everything connected to the Orbi, or (b) put the Orbi into Access Point (AP) mode.

Hope this helps