Yes, compatibility, absolutely. Just considering the 7321. It is EOL. Final firmware is 9.12.3 MR3, might not be an issue presently but you cannot upgrade ZD beyond that firmware without losing all the 7321's you have in your config, they will not work!
ZD latest firmware is 10 so you are already potentially missing features/fixes that might or might not be applicable/desirable for you. You will fall further behind the latest firmware path over time.
Edit:Just checked 7352, 7372 are supported in ZD f/w 10. So 7321 will be your Achilles heel.
Your choice now could easily haunt you in a year or two so think hard about buying old kit.
Yes you can connect it. At present no firmware problem because R500 is supported in latest firmware (and will be for a while yet as the R series are "relatively" new).
Every firmware release for ZD has release notes including a compatibility list. So for instance firmware 9.12.3 MR3 will run 7321 APs (and any others including R500 on compatibility list). If you upgrade to any version higher, 10 for instance then the 7321 will no longer work (but others such as R500 will).
As comment before newer APs provide newer features/tweaks that might only be accessed/available through newer firmware. Remaining on older firmware to utilise old kit will almost certainly limit the capabilities of newer kit. Bit of a balancing act.
It is always the oldest/EOL APs you have that might drop out of compatibility.
The firmware release notes are crucial to read if you are concerned about what might or might not be supported if you upgrade your ZD to another firmware.