Hi,
I think you can. But recommended, no.
Talking with one of the Aruba folks, the controllers are sized for the number of APs they will be processing/terminating. More APs means more CPU time for the controller. Basically, when they have 4 times as many RAPs as CAPs, they assume the RAPs will be using split-tunneling. Thus, less processing for the controller. Unless the APs are very underutilized, you'll likely to get performance problems.
Is there a specific feature set your looking for that only the controller will do? Perhaps you could get all Instant APs and eliminate the controller altogether? I believe the 16 IAP limit is gone now. Also, if you ever need to convert to a IAP to a CAP or a RAP, you can. You should talk with a Aruba SE for any caveats with the IAPs.
-Tim