Mixing races and out of contact

[dbt1949]

I just went and sealed off a system (closed the warp points)from it's parent empire and then use ground troops to conquer all the planets(something like 8).
It's been a few turns now and my empire is completely cut off but I still have contact with the empire who's planets I conquered.What's going on here?
Also how does it affect anything if I add my own population to a planet that was conquered and has it's own population?
Thanks!