Memoirs of a Spacewoman (1962) by Naomi Mitchison also contains two-stage aliens; in this case caterpillars/butterflies, but doesn't fit much else of your description.
The Spacewoman in question is a scientist and explorer. The book is set many centuries in the future, though no dates are given. Humans have explored many worlds in a number of different galaxies. Their quest is for knowledge and to be helpful: there is a strict rule against 'interference'.
The narrator, Mary, is a specialist in 'communication' - a kind of telepathy. She operates in a society where women are just as likely as men to be leaders, though she herself prefers not to lead. They also do pretty much as they please as regards sex and reproduction:
I think of my friends and the fathers of my children. I think about my children, but I think less about my four dear normals than I think about Viola. And I think about Ariel.
Viola is a haploid human, conceived as a result of sexual interaction with a Martian - Martians being hermaphrodite. Ariel is an alien creature that could be seen either as a parasite or a kind of child.
There are no space-opera thrills and fights: rather there are a number of different worlds with interesting biologies. In one case there is an ethical problem concerning some caterpillars and butterflies, both intelligent. Attitudes about sex and about alien cultures are similar to those of the New Wave, though it is not normally classed as part of it.