FISO
DCS, in my experience it depends very much on the ATSSD Inspector. At my unit we have had a requirement to replace our ATC with FIS and, on such occassions, we have to NOTAM the change in service and reduce traffic levels. Also we were told that the services could not be mixed durring a day, we were only permitted to use FISO's at either the beginning or end of the day.
As for pay that really does depend upon the location of the aerodrome.