I'm wondering if resque-scheduler needs a resque instance to run the jobs in a specific queue, or it's mandatory for resque-scheduler to use the
Yes, you definitely need to have a resque worker running to process the jobs, with a command like this:
QUEUE=* rake resque:work
Resque-scheduler puts the jobs on a queue when it's time for them to be executed, but doesn't execute them itself. That's why you only run one instance of resque-scheduler, and as many resque workers as you need to keep up with the jobs.