The way the piece was written, I put lots of breaks in it--places for people to rest out. Sometimes, I have alternated sections--sopranos for a bar, then echoed by mezzos, for example. This actually creates learning confusion. With a cappella it appears that "the more the merrier" is the best policy. If everyone is singing frequently, it enhances the intonation and everything sounds more assured.
Live and learn. And edit. Edit!