Sacred Haven of Debauchery

Another stupid question, if I've already settled on a planet, does that mean I can't terraform it? I couldn't find anything about it.

If you're playing version 1.6, there's a technology that allows it. It wasn't introduced in earlier versions.

Added 3 minutes later

and they also cut down all the resources, except for minerals.

Once you research synthetics, they'll also provide a good boost to energy and research. So, in this regard, there's nothing wrong with having a planet full of robots.