I've not raised in H. spadix or S. mesaensis, nor heard of anyones experiences, so I can't state their potential molting problems.
The S. mesaensis has closer keeping conditions to H. spadix, than H. spadix does to H. arizonensis, if you look at their habitats.
H. spadix should require cooler temperatures, and a little more humid than either. Though I need to look some locations average rainfall and a few other things to confirm this, so for now consider it conjecture.
I think Aztec's over all point is that many scorpions will have a hard time molting if kept incorrectly, and most should do pretty well if kept fairly correctly, which is correct in my opinion.
H. arizonensis is the desert hairy btw.
*Edit* From a quick glance, I will say this: While H. spadix does inhabit some extremely dry areas, similar to H. arizonensis, it also inhabits some fairly humid environments as well. I would think a small rise in humidity would be in order for H. spadix, especially if you know it came from one of the more humid environments.
But as AZJohn has mentioned, even in H. arizonensis, they seem to have access to more humid areas in their burrows than we would believe in some of the desert they inhabit.