Hi,
You can deploy 70XX controller as a master controller but it is not recommended due to it';s hardware limitations, these controllers are designed for branch office deployment.
A master controller should have rich of hardware to process ARM data received from all the AP across the deployment and Master also need to analyse the IDS data and take IPS decisions along with regular controller responsibilities, more importantly Master should have high availability with redundant power and enough cooling system.
72XX controllers are designed with enough hardware to handle and process massive ARM and IDS data and having Redundant power and sophisticated cooling system, therefore 72XX are highly recommended for Master role.
where as a Local controller is suppose to handle client traffic ( in tunnel mode) and it should map the policies along with AppRF for this we don't need very rich of hardware. 70XX are designed with less hardware such that they can handle client traffic and AppRF, therefore these are recommended for Local operation.
more importantly when master is configured as the backup LMS for all the APs, Master should have very high AP handling capacity, 72XX can handle >=512 APs and 70XX can handle <=64APs.
coming to the scalability at the BO, if you want to deploy more than 64 APs, deploy another 70XX controller or terminate some APs on 72XX ( Master).
Hope got some more clarity on this,
Please feel free for any further query on this.