No additional licenses needed with IAPs. The maximum tested according to Aruba is 128 in a cluster. You should be good i would think. Once the new APs join the cluster (automatically by default if on the same VLAN) they will match versions with the VC (Virtual Controller) This guy gets elected, or you have a preferred AP set to act at VC).
And just an FYI, you can have mixed models of APs in the same cluster as long as they are both able to run the same version of code. You can check code versions per model on the Aruba Support Portal.