TRANSACTIONS_PER_ROLLBACK_SEGMENT can not be used to limit the number of transactions per rollbeck sagment. And it doesn't matter if RBs are public or private.

This parameter is there only to help the server to calculate how many rollback segments it has to open upon startup, based on the TRANSACTIONS parameter. But it doesn't impose any limit in the number of transactions each rollback can actually handle.

AFAIK there is no way you can limit the number of transactions per rollback segment. Unless of course you have only one rollback segment online, whereas TRANSACTIONS parameter impose that limit.