At my carrier, there are no rules along the lines asked about; the rules that apply are generated by the Schedule Planning department, and will change as they see fit. They produce many schedules each year, and how much "this" one differs from the previous one depends on lots of factors regarding fleets and markets and priorities. Some things are seasonal, many aren't.
The change-over dates are similarly "whatever makes the most sense" as the schedules are being built. One carrier may be on a 6-week cycle, with another on a calendar month cycle; ours seems to change at not entirely regular intervals. Some schedules are longer than others - mid June thru mid-August may be one fairly constant schedule, but November & December may see 4 distinct schedules, as travel fluctuates around the two major holidays.
Increasing automation over the last 20 years has probably contributed to this variability.
There may be some "sacred cows" in the schedule, that flight 1 always is the first departure from (big east coast hub city) across the country to LAX or SFO, perhaps, or flight 711 always serves Las Vegas (dice roll of 7 or 11 is a winner), etc. But much of the schedule is, whatever makes sense to the planners this month or this quarter or this year.
The closest you'll come to having predictable guidelines would be to study one airline's schedules over time & observe enduring patterns... but even those may not be written in stone.
