The lesser ones are, but the Bellion stuff is being massively exaggerated. I'll post later in a spoiler box.
EDIT: SPOILERS! READ AT THY OWN RISK!!
First of all, Zoria (a
Celestial) can slice
Red Demons in half and cut the fingers off amped
Gray Demons, though he was overpowered. Dahaaka alone beat him and killed multiple of his soldiers, each of whom were putting up pretty good fights against
Red,
Blue, and
Gray Demons, albeit in groups.
Pump survives a massive beating from Ban and blocks a few of his blows. But Ban has been shown to be much faster than Meliodas despite being many times weaker, and Pump did harm him despite the latter having tanked blows of
this magnitude. However, Ban does (literally) crush him fairly easily.
Galla's dummy (who is implied to be weaker than Galla herself) destroys the entrance (a lake of sorts) to the Sky Temple, which I calced to be Low-End
Small Town level (roughtly 1.5 kilotons). She is unable to harm Heavy Metal, but can survive a Devil Hammer from Diane and Bumblebee from King. She dies right after due to a strange "Cross Shooting" combo from King and Diane, it's like a barrage of energy blasts. Galla also had one even clash with Zoria, but take that very much with a grain of salt since it was for 5 seconds, albeit she did say he's "worth fighting".
Aside from some hax (defensive spells against the elements and elemental manip), Derocchio doesn't matter. Atollah was easily petrified and only has some advanced Soul hax.
Bellion fights on par with Meliodas for multiple minutes, lands effective blows, tanks hits, clashes evenly, reflects a sword blow, overpowers him at multiple points, and would have killed him while the latter was impaled with a blade that's effective against demons. In their second fight, Bellion has an amp from Baruja, allowing him to fairly easily overpower a Meliodas clone (thought the fight was sort of even, but honestly, it seems like an outlier). The three others show up and punch him into a cliff, but that's all, he even says "No matter how many of those weaklings you can produce, you're still inferior to me", and even shows it when Meliodas has to re-absorb his clones (he has to use it again against Baruja) and use his Demon Mark to overpower the former's attack, smash his blade, and win easily.
So I'd say that the lesser knights are
City level for being superior to Solaad's dad and fighting somewhat on par with the Sins, and Bellion is
Country level.