I would say that the scheduler should not run on more than one of the instances that use the same database. The windows service may run on all machines if you like, but you can edit the configuration element enableScheduler in the siteSettings element in episerver.config/web.config to only be set to true on one of the instances.
For the second question I have no idea off the top of my head.
If a EPiServer website is hosted on a web farm, do you just need to run the Scheduler Service on one of the web servers (given they are all connected to the same DB).
Also, can you get the service (I guess via the EPiServer.SchedulerSvc.exe.config) to talk to a specific server in the web farm?
Any help appreciated :)