How vital is the rule about the number of instruments?
I read that quartets have 2 violins, 1 viola and 1 cello; I read that quintets have 2 violins, 2 violas and 1 cello; etc. But... must they? What if a quartet has 1 violin, 1 viola and 2 cellos? What if a quintent has 1 violin or viola, 2 cellos and 2 doubles basses?
Whatever your answer is, does it also apply to ensembles of other genres (rock, jazz, etc.) and/or groups (pianos, winds, etc.)?