the ideas department of my brain went into hyperdrive mode while describing that bug, so i really wanted to put this somewhere

Peaceful immigrants, peaceful natives: (idk any movies where this happens)
The immigrants & the natives immediately form a new star nation, whose governing ethics are a mixture of the immigrants's and the natives's. The planet gains +1 pop of the immigrant species as a citizen.

Peaceful immigrants, hostile natives: ("The Day the Earth Stood Still" scenario)
The native aliens kill or enslave the settlers, and take their technology. Their civilization immediately rises by 5 ages (Renaissance or earlier), rises to the Early Space Age (Steam to Atomic) with (20/40/60/80 + initial advancement progress ÷ 5)% advancement progress, or becomes a new star nation (Early Space Age). 50% chance of gaining +1 pop of the immigrant species as a slave.

Hostile immigrants, peaceful natives: ("War of the Worlds" scenario)
The planet becomes the capital of a new space nation, with the settler pops as its rulers and the natives as slaves. Planet gains +1 pop of the settler species.

Hostile immigrants, hostile natives: ("Indepencence Day (1996)" scenario)
A toss-up between the two “hostile-peaceful” encounters. Given that the primitives are at the Nth age, with the Stone Age as the 1st age, it is a (10N – 5)% chance of the “Peaceful immigrants, hostile natives” result, and a (100 – that same number)% chance of the “Hostile immigrants, peaceful natives” result. Additionally, the planet will lose a random fraction of its native pops.
(yes, this means there is a 5% chance of the FTL colonists getting killed by stone-agers. hey, they may have been particularly stupid colonists)