Really nice report and nice to watch!
I thought you cant have more Cerastus Knights than regular knights. So 5 > 0?
Technically it's fine - each of them has the rule:
"owing to their rarity in the 41st Millennium, you may not have more Cerastus Knight-[variant name] in your army than you have Knights of other kinds."
Meaning that an army could have a Knight-Acheron, 2 Knight-Lancers and 2 Knight-Castigators, for example.
The restriction is by class, not by Cerastus knights as a whole - so there are 2 castigators versus 3 knights of other types, 2 lancers versus 3 knights of other types, and one Acheron versus 4 knights of other types.
What youldn't be allowed is to swap one of the lancers to a third castigator, as then there would be three castigators versus two knights of other types, which is not permitted.
The rule doesn't limit the number of cerastus you can field, it just forces you to take a nice mix of different types. Since each cerastus was a diffent design, that's fine.
The only legality issue was the Knight-Atropos; I think (at the moment) that they're Horus Heresy only, like the Glaive superheavy tank.