Does it make sense to colonize planets in enemy territory?

I started my first game in a small map and ended up with five enemies completely surrounding my territory from the start. I found habitable planets within their influence area and thought the borders would change if I colonize them. Apparently I was wrong. (The planet does not border my influence area directly, and it did not generate influence area by itself.)

The planet now belongs to me, but is within enemy borders. As far as I read this will likely lead to a rebellion soon.

Is it always a bad idea to colonize planets in enemy territory? What tactics can I use to colonize more planets if almost all of them are within enemy influence area?