The following is a post by Michael Mauldin, in response to a composition student's question about vocal range for cambiatas, boy choirs (and men-and-boy choirs):
Again going from the bottom of your list up, the cambiata (changing or recently changed) voice is quite a strange animal. Much has been written about it, but the effect may vary widely from boy to boy. The middle stages of the transition are usually the most difficult, when the famous cracking and yodeling takes place. Strangely, at first, some changing boys will keep their high notes for quite a while, as they suddenly gain new, low notes and experience a gap (no sound, just air) for the distance of four or five notes, up from about middle C. Even though there are ways to help them cover this gap (taking the "head" voice DOWN over it lightly, and supporting well to take the "chest" voice UP over it), for such new "tenors", I would avoid writing in that spot, even though a "real" tenor can sing well up to an A or Bflat above middle C. What I did was use my cambiatas on the "bass" part, really a baritone part--rarely going much below the C below middle C. Then, I used my unchanged boy-altos for the "tenor" part, being careful to write it no lower than the F or G below middle C (a nice effect when you combine with men tenors or even women altos on the tenor part).
Of course, you said you were writing for a men and boys choir. I imagine you already know about the range of men tenors and basses. But still the info above might enable you to write, as I say, a piece that would work well in a mixed SATB, a men-and-boy SATB, AND a "boychoir" SATB (maybe more like SSAT). But what you probably need to know about the most is the unchanged boy soprano and alto. Both have a marvelous, rich "buzz" effect right around the G above middle C, up to the C or D above middle C. Then there is the bright, incredible sound (still "angelic" in the English tradition) that you get from that C on up to A or Bflat above the staff. When a trained boy choir goes beyond the staff, it will literally rattle the windows in a huge room. Not because they are shouting or forcing. It's just the penetrating effect of the overtones in that range, strengthened by the larger size of boys' sinuses (one of my choir moms used to say, "they have resonance where their brains should be"), giving them two chambers (mouth and head) in which to enhance the sound.
A boy-alto will get too breathy and not be heard well below the G below middle C, whereas most boy-sopranos do the same at or below middle C. A boy-alto can often sing almost as high as a soprano, but for comfort and pleasant effect should probably not go much above the G above the staff (a 2-octave range). Likewise, even though boy-sopranos can sing high more easily than women can, I have never written above the B-flat above the staff for the whole first-soprano section, though a first-soprano boy-soloist can EASILY hit a high C (also a 2-octave range).
--Michael Mauldin, Albuquerque Boy Choir