Depends on the view.
See, encountering a sentient race roughly on the same tech level as we are, that happens to be not too alien to communicate, and can survive in a highly aggressive and rare gas like gaseous O² is a chance going strongly against 0.
Yet you find them in the hundreds in aurora, and they also mostly breath and walk on legs.
First off, you are generally right.
Still, not to prove you wrong, but as to show you that all of it has the same likeliness as anything else, I'll answer your answers:
1) Size: Thats not necessarily true.
Poisoning a bigger creature, then running away and coming back later for the meal is a quite valid strategy. Sure it wouldn't work, but it's not intelligent. Also, what is poisonous to us might be readily available and not even meant to be a poison on that world.
A Human could probably spell disaster on another planet by exhaling.
2)As for parasites; Yes, thats only an example. What did they live from in the first place, until the Genetics on the planet support extremely volatile mutations. I agree it is unlikely, but if a parasite (which is the unlikely part) would ever be able to use a human body, for example, it would be impossible to diagnose it until it actually developed a few times and was studied.
Theres still parasites from tropical regions today that no real cure exists for, but they luckily happen to be rare, and often not kill.
3) AS for the weather, the conditions on a planet can quite render weapons ineffective, like, making long range communications and guidance impossible, or wrecking electronics. So you'd have to pack the spears again.
4)Unrest is of course variable. but if you have a small digsite, with heavy military presence, and a single group of security robots emerges and wrecks the half economy I could see people demanding a stop to the digging. Wildlife allows for more variable reactions.
5)"Political Realities" are not applicable to other races. It is likely, if we ever find other sentient beings, we will have as much mercy for them as the the ants that constantly walk over the terrace; They don't have the same genetics, hell, they'll have less resemblance to what we would call a "living being" than lichen or microbacteria. The first of which btw are a synergy lifeforms, which is an interesting question if your hunting for Hives, which might be mobile. And probably look like the other 5000 trees around.
Which leads to another challenge: [b]Ethics:[/b] Different races will have fundamentally different views.
Just imagine an energy based race whose physical appearance is a mere shadow of themselves, consisting of Ionized gas.
They might consider Gaseous weapons like Flamethrowers, Bioweapons etc. as quite normal, but feel strongly against using projectile weapons whose metal projectiles would probably suck off the electrical current like an acid cloud would dissolve us, only penetrating and faster.
6) You'll probably only need them first to overwatch the installation. Sadly Aurora doesn't support that kind of stuff.^^
Last but not least:
As said, it is just as likely as any other combination. [b]Roll a lot of dice.[/b]
Insects and bacteria have those kind of behavior already, btw, Insects will die to defend their populace, as they themselves will not breed, a few select do that. A bee won't survive aggressive contact with a human, but it doesn't care for it's life if it's for the greater good.
Seeing how it will be completely impossible to kill all potentially hostile insects on this planet, short of a very sophisticated Bioweapon (theres a fungus that is slowly killing of all amphibia on earth atm, with some back luck were rid of them in 50 years), those can prove to be a threat.
Military is used as a way to simulate the engagement, it would probably be engineer brigades for any non-direct combat. Like, deploying Bioweapons, you will want to have that under military control.
For the growth: Some Bamboo can grow up to a meter per hour given enough nutritious ground, so growth of not too intelligent creatures could easily be that fast if they are successful enough to justify it. Which colonists could be, maybe the resources on the planet are relatively scarce, and colonists add a never expected addition that allows half the wildlife, including plant spores, to feed on them and grow way faster.
In which case one could actually kill the wildlife by dropping a few hundred criminals, waiting a month for everything to massively grow, use up all natural resources and die, and then drop in again to get rid of the rest.
But why am I writing this, I agree with you that it would be quite unlikely.
You can now again answer to my answers, and so on, we wouldn't get ahead.
There was a thread, maybe in chat or somewhere, about the likeliness of breathing, and it kinda didn't end well.