Hi,

Those rollback segments are tiny.

Oracle recommends initial and next extent sizes to be of equal value, I would recommend you set it to about 2M to start with.
Minextents should be set to 20 to avoid dynamic segment extension.
optimal should be greater than minextents * next
The amount of rollback segments created should be decided using the formula.

transactions/transactions_per_rollback_segment.

initial and minextents cannot be altered for any segment once created. You will have to drop and recreate your rollback segments.

Firstly ensure that there are no active transactions in the rollback segment you wish to drop by querying v$transaction. This can be joined to v$session to establish which session owns that transaction.

Once you are sure the rollback segment has no active transactions issue :

alter rolback segment rbs_name offline;
drop rollback segment rbs_name;

now create using.

create (public) rollback segment rbs_name
tablespace rbs
storage (initial next minextents optimal ... ..)

query v$rollstat, v$waitstat, v$sysstat to query how your rollback segments are performing.

Suresh