Predatory trait can't force a yes vote; the only things that can are Helps Allies*, being the vote initiator (as in party to the relationship change over the -50 threshold that triggered an alliance vote), or already being at war/peace (depending on vote type). Other than that, 200k war weariness should easily swamp all other considerations.**
The bigger problem is probably diplomacy events dragging factions into wars they don't intend, but I'm also not sure the outcomes are actually bad in practice; Sector might be too static otherwise. Maybe they could be less frequent, at least; I see that with strategic AI off the mod allows intentional negative events if war weariness is above the no-war threshold, gonna change that.
Otherwise, TBH this is the kind of problem I'm more likely to fix by just changing the weariness number.
*I don't presently intend to change this; my current position is that traits should do what they say on the tin barring a compelling reason otherwise
**
WEARINESS_MULT is 0.005f, so 200k weariness gives 1,000 points towards preferring peace (while the requirement for a yes vote is 100 points). The partial calculation is printed to starsector.log for any faction that hasn't been forced to a yes/no vote by other factors.
Is there a means to edit the selection of a starting system? Like only in systems with X amount of planets, or only on worlds with the Habitable trait etc.
Not at present; Kaysaar was working on a feature for a fully customizable starting star system, but has been busy lately.