I think there is some confusion over the composition of a Database Cloud Service. This Service is not an instance - it's a slice of an instance, implemented with schema isolation. So just as you could not create a RAC node from a schema, you cannot create one from a Database Cloud Service. In addition, configuration options like RAC are not exposed in the Database Cloud - it's essentially Platform-as-a-Service, not Database- or Infrastructure-as-a-Service.
As far as performance goes, you should refer to the white paper on performance, available here - http://www.oracle.com/technetwork/database/database-cloud/public/performance-wp-1844129.pdf - to help you better understand how performance is managed for the Database Cloud. There is no limit on the number of users you could have for a service, but please keep in mind that it will take some amount of time to add that many users to whatever authentication method you choose, so a "short period" is relative here.
Of course, the good news about all general performance questions is that they all have the same answer - "It depends". An Oracle Database 11g R2 instance running on Exadata could definitely handle that many users, but the performance delivered depends on what they are doing and what everyone else in the shared resource environment of the Database Cloud is doing. The white paper should help you to address your particular situation better.