Great question!

I would say that it depends - I would personally not mix them in one hymn, but possibly use them for a prelude/postlude and obbligatos/descants. It sounds like an interesting ensemble to write for. I assume that there is an organ available. What grade level do they play at out of interest?

The alternative is to ask them to sing instead/"in alternum". I would never turn musicians away because they can usually read music, have trainable ears which is a great help especially when the choristers do not read.

Heed the call, but do it well - this takes practice and humility from the musician, and a patient choir master/mistress :)

PM me when you have a moment.

The violin and flute would be appropriate together, as well as separately, for the hymns and possibly chants that this forum represents. If a motet needs a solo instrument, use them. I've used both the flute and the violin to intone acapella music, as well as play descants when I do not have the singers.

The Sax, however, poses some problems for me. It's rare (at least where I am) to find a sax player who is not trained in jazz but in classical music. This goes for most chants, hymns, and motets, which are written in a classical style. (I am presuming that you use at least some of the traditional music this forum focuses on, if not, then this part is not as relevant) You could use the sax during mass, but it may clash with the organ. If you are using flor y canto or another OCP resource, then there are songs with specific Eb Alto Sax parts, I would suggest using those. Otherwise, save the sax for prelude and postlude.

Encourage them to sing, they sound like they might be good readers.
You CAN do the church sonatas with the others. Use the trumpeter for weddings or special occasions, get them to play te deum etc. actually there is heaps of Spanish trumpet music for the liturgy out there that is from the 1600's.
Avoid weekly mixing of all three parts.
I would avoid getting them to play hymns only because I'm not very pro too many instruments in mass.

@Jes
"heaps of Spanish trumpet music for the liturgy out there that is from the 1600's"

can you point me in the right direction?

@Lars definitely look on IMSLP for scores. You can use multi-category search for this. A reminder that you can use secular works if they are not being sung and also to double check if the work uses Trumpet in C or Bb or other transposition before print and play.

With those colorful timbers in close simultaneities producing all those extra high frequencies, perfection in intonation should be auditioned prior to service to ascertain the musical usefulness. You wouldn't want to add anything negatively distracting, and neither should the musicians

This right here. I know it’s an old post but it’s worth repeating. There’s nothing worse than sitting down in the pews during Mass and trying to gracefully sit through instrumentalists (and soloists/cantors, for that matter) who have poor intonation and spend Mass fighting off outwardly manifesting the pain and discomfort of poor intonation, sound quality, etc. It’s so exhausting that I need a nap after Mass.

I'm not sure where the sax would fit in

It really doesn’t. I say this as someone who plays tenor sax. I’m not anti-sax. I love me some good sax, but I’ve been to Mass where there’s been a saxophonist, and it just doesn’t work. The timbre of the saxophone just doesn’t work in sacred music. It’s just loud, gritty and raw and doesn’t blend. You might be able to pull off a soprano saxophone if you have a professional with good tone quality and the ability to blend, but anything else is going to stand out and dominate because of the distinct sound of a wooden reed resonating through brass. Then there’s the issue of less experienced players and their reed squeaks as squawks because they try to control their tone quality by biting down on the mouthpiece.

Encourage them to sing, they sound like they might be good readers.

This seems like the most obvious thing to me. As an instrumentalist myself who started singing in our Latin Mass’ choir and schola without really any previous actual choral singing, years playing scales, arpeggios, thirds, etc, and practicing with a tuner made sight reading singing polyphony rather easy. What I thought was me being wrong, was just my assumption that since I was the newest member, it must be me who is wrong, off pitch, etc, but as it turned out, it was those who didn’t really have any prior music experience and weren’t being corrected when they were wrong. Turn your instrumentalists into strong choral singers. It will also improve their playing as well.

I had a friend in a Protestant church who had a choir lady who played one of those nasal, medieval instruments

I’m holding out for someone who plays the hurdy gurdy, which was apparently the instrument used during Mass before the pipe organ was given pride of place at Mass to support the singing. I’m noticing a trend of preference for instruments that can continuously sustain a note indefinitely.

@mamaherrera

You might find some great resources here too:

https://domenicozipoli.org/