You can assign a custom ringtone to a contact without changing other notifications.

Custom ringtone options appear with your contacts and your notification profiles. When you assign a custom ringtone to a contact, a new profile exception appears with your notification profiles as "Calls From <contact>". Notification profile exceptions are shared between your contacts and your notification profiles and changes that you make in one application are made in the other application.

Your BlackBerry® device bases the volume for custom ringtones on the volume that you set in the active notification profile.