No rules, WB. Except maybe a guideline - not so much a rule:
Instead of immediately focusing on arrangement issues - as this thread seems to do - why not first step back a couple of paces and ask yourself: 'When I imagine developing original material for this group, what is the sound I hear in my head'?
Before you commit to addressing
any of the implementation issues, first
fully and completely explore the musical "vision" you have for the ensemble - and develop
that.
Only once that has visualized (audiolized?) as completely as possible (at least for a first round),
then turn your attention to the matter of 'How do we best translate the concept into finished, polished material'?
Get the horse before the cart. Make sense?
MM