I'm pretty sure like/dislike of government type is based on their government type, not their race. I'm probably wrong.
Yes, this is absolutely right. Jacob/Lee described it very well.
IMO, the relations malus based on size and power needs to be nerfed or be dependent on how aggressive/underhanded you are. If you're powerful enough, other states instantly dislike you, no matter how benign or passive you are. A baseline level for jealously is reasonably, but, as a respectable empire that never started wars, I was seeing -20 relations immediately with anyone I ran into. These races didn't even know the size of my empire or my population and they instantly hated me for showing up and saying "Hi, guys, we're out there.". Then a long slog with gifts and ambassadors for them to say "Oh, you've got lots of cool goods, why don't we trade?"
Also one thing needs to be mentioned - every new race you meet WILL automatically have -20 or so relationship modifier due to being "confused by your strange, alien ways" or some such thing. Fear of the unknown or whatnot I guess.
And on topic of insectoid races - it has to be said, there is a bit more flavor to them instead of just "insects = evil". The main thing is all of them have "aggressiveness" level pretty high. I kinda makes sense, if we think about them just as being very expansionist oriented. Also trustworthiness or something is usually a bit low. AND let's not forget they instinctively hate humanoids (and rodents?). But you can get along them quite well. IIRC Sluken can be quite reasonable, but much to my surprise, in the last game I played, Boskara very more or less cool with me too. There are few factors here really - with galaxy being a collection of clusters, our empires were situated in different clusters, thus immediately cutting out the "covets our colonies/resources" malus. Then I was also careful not to expand in their cluster once the technology allowed that. And there was some trade going on between our private sectors. Also, our empires were quite the same concerning size/military strength and occasionally I slipped a small monetary gift under their table... Yeah, in the end I actually wanted to provoke war, because we both were mere digits away from achieving victory and all I had to do was have an excuse to reduce their population a little...but being Teekan, I already started 1 or 2 wars and that was dragging me down enough to be weary of striking first. In fact, I still have to finish that game.
So yeah, insect are not really that bad, just a bit more opportunistic. But so are some other races.
I'm looking at you, Haakonish. >:L