You should open a Service Request to validate if Role separation accounts is supported in Exadata.
According this White Paper, you can. How ever, be sure where you want to be, and what will be fully supported.
Have a look at this
Well. That's a very common question that I used to rear in ACS support.
Oracle has created OFA, which is dedicated to this. (Oracle Flexible Architecture), with OFA you'll use "oracle" user for multiple Oracle Homes. You'll need only to export ORACLE_HOME variable to the OH you wish to connect to. There is no need to create a bunch of users for distict OHs. However, you can do that way with no problem... its your call....
On a non-ASM setup, I can see why you want to have different OS accounts for each database. For example, one of my customers wanted to refresh one of the test databases and run "rm -f * " without realizing that he was in the wrong directory and all databases were deleted. If the customer had separate OS account, he would be safe.
But on ASM where DBA don't really use OS commands to interface with Oracle, I see less reason why we want to have separate OS accounts.