Hi Mohan,
IAP internal user DB will support upto 512 users Please make a note, IAP-9x supports only 256 user entries. If there are already 512 users, IAP-9x will not be able to join the cluster
To form a cluster IAPs should be in the same broadcast domain hence it doesn't matter whether it is a manageable switch or unmanageable switch.
Coverage holes all depends on your deployment . don't forget you have IAP mesh point option available.
Please feel free for any further help on this.