Off the top of my head, I always thought it was meant to go somehting like this:

Mini buses in 3 digit series (3xx for the MetroRiders, 4xx for the original solos, 5xx for the original MPDs etc)

The 4xxx series and then 5xxx series was for full size single deckers

The 3xxx series was for double deckers, until they ran out and went in the 6xxx series.

Coaches in the 7xxx series

Midi buses (darts,versa etc) in the 8xxx series.

I agree, Stagecoach are the best at this.

For some reason I find myself annoyed by GNE's insistance on sticking fleetnumbers anywhere. Windscreens, doors, windows, panels, upstairs window, cab window etc