Much human conflict comes down to primate dominance dynamics. The question isn't "could they get along" but rather why would they fight? Conflict is a resource investment that needs to pay off. There's no reason to fight if there's nothing to gain. In humans there is always something to gain because everyone can use everyone else's resources and mate with everyone else's mates. Interspecies, not so much. But there's nothing we'd need from dolphins (or sentient arctic seals, or sentient subterranean creatures, etc) that isn't purely an economic thing. At that point there isn't much contact and it's just cheaper to trade.
Personally, I've seen enough wholly irrational maliciousness/fear/etc. to make me less sanguine than you on this issue. Even if there's nothing to gain (or indeed everything to lose) from hostility, hostility could end up being our collective response anyways. I'd be happy to be wrong on that point, certainly.
At least the portraits totally confirms the ability to have a mushroom with a mohawk and some bling.
"I pity the fool that sautées my brothers!"
Some of those mushroom folk are awfully, er, wobbly in their animations. Would make modding mohawks and the like more tricky, I imagine, but there's probably some passionate artist out there eager to make an "Immersive Mushroom Mohawks" mod.
Actually, are the 'lesser leaders' animated the same way the head honchos are? Not sure I've noticed in the videos thus far.