Thank you cjoseph for the link. This confirms that the solution I'm looking for should be possible.
The other concern I have is that the joining of new APs seems unstable in a mixed environment.
For example. I have a network running with 20 IAP-115 and connect one factory reseted IAP-225 with two ethernet cables to the network under the exact same configuration circumstances(it's a extreme networks environment) as the other IAP-115s. Resulting in a crashed network where every IAP-115 in sequence stopped answering to ICMP and in time loosing function all together. In the end there was only one IAP-115 master left. The only way to start up the APs again was to manually power of the PoE for each and every AP and start them up again. The IAP-225 had the same version as the others, it got the configuration from the cluster but it never started up properly.
Since this accident I am hesitant to very hesitant to mix different IAPs in the same network.
Also I'm wary about the reliability of the Instant cluster. Is it reliable enough to deliver wireless to a office that is in its entirety mobile.