IMO you need to get into one accord with the music leaders. Listen carefully to their input; that helps them feel valued. Then discuss. If they don't see a clear REASON for things like vocal training/warmups, separate practice for singers + musicians, or any other commitment to excellence, they will not support or follow it through and those efforts will backfire.
And please take care in how guidelines for any group are phrased. For some, saying that something is 'mandatory' is like waving a red flag in front of a bull...big difference between saying something should be done because it is good and laying down edicts to be obeyed (or not)...