Hello
You can roam between cisco and aruba if you have them in the same vlan and have the same default gateway and the same encryptation
Anyways
the problem could be the PMK caching or possibly OPMK caching aruba has turned it off by default turn it on the controlller and issues like this were resolved when you enable it.. the thing is that on instant you cannot change this value(an i really dont know if this value is on or off by default on instants APs)
The other thing i saw is that you got 45 IAPs on the same VC
Take in mind that its limited to 1024 concurrent connection per group!.
As reference of this last thing
http://community.arubanetworks.com/t5/Aruba-Instant/New-Instant-AP-SW-Released-v-3-1-Sept-17th/td-p/45314