Can one use chocolate fèves/callets/nibs for tempering?
Yes, you can temper the callets.

Couverture is couverture, and the only thing that's really different about the callets is their shape - they're specifically intended to be melted easily, unlike the bars which they (sort of) expect people to eat directly.  The bars are less likely to melt during shipping which is why they also sell them in that form.