My understanding of the Goo role (note: i have never played a game with the role until now) is that it is a mostly reactive mayhem role. Something a mod adds to a game to create mayhem. As you can see, there are a lot of ways the goo can cause mayhem - from blocking people targetting it, to poisoning anyone targetting it.
Per the rolecard flipped, Listo would get a random color every night. Depending on his color, one of the effects would happen.
So f.ex, had Listo been a blue goo when he was lynched, Largehorn would now be a blue goo. That's a cult right there.
The goo role could also be a passive cult recruiter, if black color. Also, the role on mafiascum also has a "grey goo" power, which is straight up a targetable cult recruiter. Whomever targeted turns into a grey goo themselves. We do not know if he started with a color, but IMO I do not believe so. I'm in no rush to lynch Largehorn over this just to be sure, at least.
So yeah - Listo was scum in role, even if he didn't have to play for scum in effect. However, I am fairly certain he counts as third party scum as far as game setup is concerned, probably taking the place of a serial killer this game. I do not expect there to be more goo roles around, unless someone else independently also started with that role.