I do know that my medium-op level 6 party would die if I pulled a Tucker's Kobolds with Goblins. Because they are silly people, and don't know to carry a means of making light with them (Tier 1+2s are banned, and no-one's got Light on their list), and don't have Darkvision.
CR 1/3s are a lot more threatening if you have them in small tunnels in the dark... Especially if they burn a lot of incense (pretty much blocks Scent), and stick to the edge of their darkvision range as much as possible.
After all, after a few javelins coming from god-knows-where, all targeting their flat-foot AC (I don't think any of the people in my game have Uncanny Dodge), which probably have Aid Another benefits, people start moving towards the darkened area.
Which is where some mundane rubble, set up like a maze through-out the passageway, comes in real handy.
Add in the penalties for squeezing in combat, and you've got the medium-sized PCs at serious disadvantage.
However, canonically, any group of Goblins pulling of that plan has bigger fish organizing them, so...
So, yeah, a buncha weak characters can, as long as the party doesn't include Tier 1+2s, pose a threat to a decent ECL 6-8 party.