Adalgisa, soprano or mezzo?.
I've been always in favor of a philological approach to answer this kind of questions.
Between the dissapearance of the 'castrato' from the Italian operatic stage towards the end of the 18th century, and the undisputed emergence of the tenor as the main male role around the 1830s, in many operas the male protagonist was a trouser role, usually trusted to female altos.
The composer differentiated in the score the trouser rol and the female protagonist, using range and tessitura, as we can easily understand listening for instance to Tancredi and Amenaide.
However, when we are talking about the 'seconda donna' in an opera with soprano and tenor, like in Norma, the differences in range are minimal (usually the 'prima donna' descends to a semitone or a full tone lower, while the top notes are the same pitch, or just a semitone higher). The true characterization was in the singing style; more coloratura and 'fioriture' for the 'prima donna', that got the higher notes in the ensembles and her tessitura is somewhat higher, too.
With the decision to use mezzos (and even dramatic mezzos) to portrait this 'seconda donna' we are very far indeed from the original intention, and while in the 19th century Giuditta Pasta sang Tancredi and Norma, in the 20th, it was Marilyn Horne singing Tancredi and Adalgisa.
I prefer a soprano for Adalgisa (though Horne, and other mezzos, were great Adalgisas, of course).
Mariella Devia and Carmela Remigio singing "Oh, rimembranza" a few months ago, in Bologna: