Hey there, welcome aboard!
I think this idea has some potential, with some work. I really didn't like SCP-####-3. It came out of nowhere, and seemed like shoe-horning a traditional scary monster into an article that wasn't otherwise about that. (I'd also like to propose that part of why spiders are scary is that they have weird unfamiliar shapes- that is, a humanoid spider is actually *less* scary than a normal spider.)
I'm also uncertain about SCP-####-4- spiders combining to make a larger spider, is… potentially doable. But the transformation there bears elaborating on, since that's weird. Can a small amount of spiders come together to make a smaller big spider?
The connection with arachnophobia isn't bad, but I think just making arachnophobics more scared isn't enough (objects that instill some kind of emotion in people who interact with them are very hard to write well.) Suppose they're attracted to them instead? Or something that plays off the hivemind- maybe it makes them smarter?
The hivemind bit is interesting. Maybe expand on that- read about collective intelligence, swarm behavior, or even grexes (when an amoeba goes from independent movement to forming a sort of roving collective group- not sure how relevant, but might have some neat concepts to draw from.) See what else you can do with that idea.
There's some ideas, I think your first steps will be refining the concept. There's some phrasing stuff too, but the concept ought to take precedence.
(Oh, and a more minor note- consider not using black widows. A, they're a very well-known spider that's mostly scary because it's venomous- but if you're an arachnophobe and there's a sentient herd of spiders, the scary part isn't venom, it's too many spiders. B, and I think a very good reason, is spiders sometimes naturally form groups, and if you find a creative commons image of a spider swarm, you can use it in the article.)
(Edit: Also, consider: If you make the spiders Brazilian Wandering Spiders (you could include a picture of one as a single instance, if so), the list title could be Bazillion Wandering Spiders. If you don't use that, I might.)
