does going to +0 pop growth and beyond cost anything now? I think I read it would kill your troops at one troop per tick, but I haven't noticed that happening. I've been at +0 for a long time, and unless I get my army killed at some point I don't believe I'll see pop growth the rest of the game.

for now I'm gaining population by razing captured outposts, and finding wrecks. Am I paying any penalty for being overpopulated?

If your overpopulated then you need to make as much troops as possible and the pop count should start again..

If your underpopulated then u need to do what youve been doing to get population and also get crystals and conquer colonies as they add to the pop count as well.

Hope this helps