at what size does it safely breed at? and does it really prefer the elevations and rock slates over 4000 feet?

Welcome!

First of all you paragraph is loaded with some misconceptions. A mtn king or any kingsnake does not need a certain weight to breed "safely" > safely is the key word here. If a female ovulates she will reproduce. Simple as that.

Moutain kings in genral can be found in elevations from sea level to above 5000 ft.
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FR quote:
"Doing the same things over and over expecting to learn something else, is the definition of insanity"