Check with Dynon Tech Support but I believe the system will work normally. The Skyview Classic and HDX run the same operating system. The firmware on the HDX moves the engine instruments to the bottom internally but I believe all external items are identical. Should talk to your Classic just fine.
I don't think this is true. Copied from Page 2-1 of the latest Skyview SE/Classic/Touch/HDX Installation Guide:
SkyView units cannot / should not be mixed
For planning your SkyView system, or upgrades, Dynon Avionics recommends planning / budgeting for installing the same “family” of SkyView displays, not to “mix” displays from different SkyView families.
SkyView SE displays cannot interoperate in a SkyView system that includes SkyView Classic or SkyView HDX displays. For example, if you plan to use a SkyView SE display, your SkyView system should only use SkyView SV-D600 or SkyView SV-900 displays.
SkyView HDX product line will eventually add features and products that are not compatible with SkyView Classic. Thus, Dynon Avionics does not recommend depending on “mixing and match” SkyView Classic and SkyView HDX displays in the same SkyView system. A future version of SkyView Software will remove the ability to interoperate between SkyView Classic and SkyView HDX.
For planning your panel for the first time, or planning to upgrade displays, the displays should be “all SkyView Classic” or “all SkyView HDX” displays.