i've heard that males grow far faster than females. i had one (died at 6" from ich) and he grew from 1" to 6" pretty fast, would have been faster but he was in a smaller tank for the time i had him, if only he hadn't died he would be going into a 90gallon.
Dont know the earliest.....but the easiest way is to wait until they are sub adults and if the pike is a lady, she will get a nice bright white stripe on her dorsal and tail....if male, the stripe will stay black.
Don't worry, they lose their shyness. I finally have the male living "somewhat" peacefully with the larger female. I'll try and get some new pics up. These pics are almost a year old.
I don't know if any are considered easy, but the saxatilis group tends to be the easiest, with the wallaci grouping in second but only due to size not ease. I would guess that there are around a dozen species that are bred with any regularity. Lugubris group spawnings are considered rare.
I'm dying to have some baby Xingu pikes born at my house. About the only thing I haven't tried to get mine to spawn is RO/DI(probably the key) and photoperiod manipulation.