Role-playing issues aside, don't you take a diplo hit for cold-cocking even a minor that way?

Well, in real-world diplomacy, you'd still take a hit with some nations no matter how small the target state. Your evil friends might admire the sudden brutality, but peaceful folks wouldn't.

But I guess I just explained myself mostly out of my objection. The game favors war. I finally finished a war win this week, but it doesn't seem to have taken me all the way through my notion that GC2 is at least as much a free-play toy as it is a competitive game. It's a bit more game than toy, at the least.

From what I've seen, you take a diplo hit for wiping out multiple races, and that includes minors.   If you look in the Foreign Relations screen, one of the minus signs to diplomacy you can get is something like, "your history of genocide".   I think you can attack as many minors as you want, so long as you don't take them out.    One strategy I've done in the past is to sell/donate them a planet just before invading theirs, so as not to wipe them out.  It's well worth the 1 planet not to take the diplo hit for your entire empire.   Just...ho hum...1 million population, no starport, no factory....  They're not gonna bother you.