I think this combo is where Moral Democracy will shine. A waste of pacifist ethos not to do so.
Every race will be happy. Even the ones you enslave.
i think stacking happiness bonuses will be better than a lot of people give it credit for. seeing as pops will rarely have 100% happiness for a number of reasons, happiness is basically an across the board productivity bump. Moral Democracy gives it for everything as far as i can tell. Your starting pops, alien pops, enslaved pops even?
you could very easily add in spiritualism for 5 to 10% bonus for pops that have not diverged away, grab communal to permanently give another 5% bonus to your pops.
its hard to say without having played the game, but i really think happiness bonuses and by extension the Moral Democracy govt type may be one of the strongest across the board. If you casually compare it to say, Plutocratic Obligarchy which is 5% more energy credits and 5% more minerals, Moral Democracy basically gives you 10% more of both of those, AND on all forms of research and more food and less faction problems etc.
this is of course without having played the game, but i feel like Divine Mandate (stack happiness from spiritualism + communal and lower ethics divergence with conformists would create a very homogenous empire within your starting pops), Moral Democracy (mass happiness) and the three military government types (blob for days) would be the most "ideal and strong" but they all look great. i'm pretty happy with the balance at a glance