As a Mass Start, you’ll only need one timing device as your start device. Using a phone or a summit will be your best choice. Accuracy is not hugely important, as it is a First Across the Line, but you should do your best to get a good time. The Starter, usually a commissaire or someone in the announcer booth will give a countdown and then the start signal. Collect your start time based off that and you are good to go.
One thing to be aware of, as it makes it hard to impossible to get an accurate chip time at the start, are call ups. In large mass start events throughout cycling, the announcer will often call out racers and have them advance from the pack to the start line. Eventually they will call everyone up to join these riders, but as any loop at the start will be centered on the start line, a large number of chips will be in the field for several minutes before the race starts.
Road Race starts can be very casual at times. The race is often so long that taking a few extra minutes to make sure everyone is ready at the start doesn’t really affect things, so don’t panic if you end up behind schedule a bit.
You should set up your race file so that each course is starting separately. Then for each course group you can enter a single racer number into the phone or Summit. This will give this time to the whole course group.
If all your racers are starting together, still divide them by distance. you can then just copy the start time and enter it for each group.