I'm getting my exotic planet today, and I want to know how to make it the best it can be. I invaded a crappy planet from an inactive level 6, and used an assimilation locus on it. Anything else I should do?

Put ALL your limited artis on it.
Spires, zolazin analysers, drannik databank (if you have one), the lot. Any 1:1 space ratio things are good.

I have no spires yet, but that is exactly what I was going to try to do. Is there any way to buy some of those with GP?

If it's Icy then use a Permafrost on it first, if it's Volcanic then use a Volcanic Heat Seeker first.

...it was a barren...should I get another?

I would go with Icy or Volcanic for the reasons mentioned in the post above.

Could have saved yourself a planet by utilizing the wiki.

Welp, I'll just drop the planet I have right now

It is really only a minor difference... 15.525 MP or RP per hour (13.5 * 10%) but if you want to get the max out of the planet, that's how you do it.

Found a small icy with a permafrost on it, owned by a level 206 with no legion, only 450 defense. I think he is trying the same thing, but not for long.

If he has no legion then he very well might not be...

Well how long has the permafrost networks been out?

Christmas LAST year

Just looked at the age of the colony. 11 months. So yep, he's inactive. Lucky me.

If you can get your hands on a DM79 from a CT Lab, it would be 6rp/1 space. A very valuable artifact. I don't know if you've researched far enough to have access to one though...

I have access to the CT lab, but not loyal yet, and not sure if they have it researched yet. If not, I'll get to work on that as soon as I'm loyal.