There are no such differences in terms of nuances that you point out. Both are not rude and can be used for 4,5 people.私たち and われわれ are always interchangeable without making the sentence rude or improper.
Instead, it changes an impression a bit. われわれ sounds slightly more academic or political while 私たち sounds slightly softer.Or I can say われわれ, in any grammatical situations, is more used by male speakers. But it's not strange at all if women use われわれ.
