My optimism says that radical consensual transhumanism might be a universal Schelling point and that if humanity reaches that point we'd fit fairly well into any advanced alien culture that reached the same values. Probably the biggest value difference at that point would be the morality/consciousness thresholds for tiers of personhood.
I don't think any morally advanced culture would be willing to accept an indefinite policy of "live and let live" between the cultures as a whole; there are some injustices (slavery, murder, torture) that wouldn't be tolerable to exist in the known universe.
But that raises a moral conundrum of interference with an alien culture’s development and impact on their world. On Earth, intervention in other societies has not gone over that well. If you have relatively equal societies in terms of technology, then you risk devastating warfare.
It's definitely a moral conundrum as evidenced by the U.S. civil war, the two World Wars, and some interventions in genocides, contrasted with the lack of intervention in other genocides or in Soviet and Chinese (to pick on the largest) mass murder/starvation of citizens for political reasons.
I agree that equally powerful societies face the hardest choice; whether to risk everything to save everyone or to knowingly allow atrocities to continue while preserving themselves.
I don't think any morally advanced culture would be willing to accept an indefinite policy of "live and let live" between the cultures as a whole; there are some injustices (slavery, murder, torture) that wouldn't be tolerable to exist in the known universe.