Yes, you can have redundancy between two different models of controllers. If the primary fails the AP will then join the secondary controller if configured. If you don't have one configured the AP will try to join another controller in the mobility group. During this time when its joining another controller the AP will not be servicing clients, so those connections will be dropped. After it joins the secondary controller they will be operational again and clients and associate again.
The AP sends out primary discovery requests to the address of the configured primary. When it sees its primary is back online it will rejoin the primary controller. Again service will be disrupted during this time. As long as you keep the code versions between the controller the same the join time is pretty short. Longer outages occur when the code versions are different because then the AP has to download code when it joins.