Quick question:

What tiles are able to stop creep overwriting them?

I've been using Stone Path, and it keeps getting eaten by creep.

It depends on Creeper settings. By default only pathing cover of 140%+ running bonus (Concrete , not Stone brick!) can stop them. But you can reduce it (I suppose it will work) to 130% to stop with Stone Path.
https://wiki.factorio.com/index.php?search=walking+speed

Rampant SiegeAI (when biters build nests/worms closer and closer in direction of your base center) is not stopped by any pathing. Pathings, rails - everything is destroyed by creep generated this way (not Creeper way). You must defeat biters and destroy enemy structures to stop them.

Oh!, That's what Siege AI does. I was never sure.

It still sends settler parties out though, right? So killing them before they turn into nests still works?

Stone Brick not working makes some sense, since it's basically cobblestone.

So killing them before they turn into nests still works?

Yes, of course. But sometimes they can do it very fast (not SiegeAI, but other AI tactics in different situations, when player kills/destroys near their nests).

It still sends settler parties out though, right?

Yes. In SeigeAI phase it does, and this phase is much more aggressively now (in Rampant2.0+), because settles a little closer and those biters parties don't change building target place when see resource patch on the way from source to player's base.