Can I add / delete machines on a planned route?

Yes! On the Route Progress page, you can hit the green button at the bottom labeled Add to Route. On the following pop-up, you may select machines that were not originally routed to be serviced on that route. 

You can delete machines from the route by click the red X to the right of the machine to remove them for the current route, or you may re-route any unfinished machines to another route. To do so, when you've finished with the route for the day, you can click the button Manually Finish and re-route any remaining unserviced machines.