To be clear; there is no "incompatibility" with any Ruckus Access Points when co-deployed with each other. All the Access Points adhere to IEEE802.11 standards and can coexist, even in standalone mode. Once a Ruckus controller is deployed e.g. ZD, SZ, Unleashed, Cloud, then the APs will work in a coordinated fashion to optimise performance.
Your issue sounds like configuration mismatch between R500s and the R720s and for some reason clients are more inclined to connect to the R500 APs. Remember in a Wi-Fi network, it is the client that makes the decision which AP it connects to.
Is your Unleashed master showing that the R500s are adopted and have the same configuration pushed to them as the R720s?