I was a little surprised to see Percy Dearmer -- known for his landmark work in The English Hymnal -- suggest in The Parson's Handbook that parish choirs be smaller than the custom then and now, and that they might well be put in a west gallery if the church has one. The chancel -- or its equivalent, if present -- should not be overcrowded. An especially difficulty since so many churches have tiny chancels to make the most of congregational seating. (This was particularly true in my last pastorate.)
Dearmer makes a lot of sense, but since choirs have a political life of their own, especially (it seems) in Unitarian Universalist congregations, I'm not sure how far his advise would go. But read for yourself.
Chapter two: "The Chancel and Nave, and their Furniture" (Google Books)