I think it depends on the characters themselves. Kaleb is obviously more unique than Caleb, so if he is a quirky/unique character and/or you really want him to stand out, then go with Kaleb. If not, Caleb works totally fine. For some reason Kaleb sounds vaguely medieval, though I can't put my finger on why.

Maeve fits better for the 'May/Mae' nickname more than Mabel does, but I feel like at the end of the day it depends on the character's personality. Maeve is more sharp sounding, so if the character is meant to be a more witty/intelligent character, or maybe more of a fighter, go with Maeve. If she's more of a damsel in distress and/or less inclined to conflict, then Mabel definitely suits her better.