I started on tenor, my vocal range is baritone, but tenor was close enough. For a soprano singer, alto would be close enough.
I just think that if they can sing some of the notes they are playing, it will seem more natural to them.
But then, I could be wrong. Although I've given lessons to a half dozen or so, I'm not a professional teacher, I am a professional musician.
If someone came to me and I thought I could help, I'd agree, if not, I'd tell them I'm not the one, and the people that came to me already had instruments.
Notes ♫