I just watched a Youtube video of Conan O'Brien and there's a comment below saying "He(Conan)'s so much funnier than Jimmy Kimmel, Jimmy Fallon, Ellen Degeneres and Seth Meyers combined.". I know that the "combined" is in fact an adjective here, and I also know adjectives can be put after nouns like "something" and "nothing", but what is the rule that allows this "combined" to be put at the end? Or like, why can you do this?
This is a good question.
"Combined" is a participle, which means it can be used as a noun, adjective, or part of a verb.
The normal rule is that adjectives precede the noun being modified. However, the preceding sentence shows one of the exceptions to that rule if you consider "being modified" an adjectival phrase. It is not an exception to that rule if you consider it as the ellipsis of of a clause (a relative clause in this case).
precedes the noun [that is] being modified
So I do not know that it is an error to categorize "combined" as an adjective: categorization is a free exercise of the human mind. You could reword the sentence and put "combined" before the list of names to create the impression of a hydra-like creature, each head of which is emitting topical quips. But I think it is more useful to think of it as being the key verb in an abbreviated clause
if their talents were combined
Following on from what you understand already, it is because...
Jimmy Kimmel, Jimmy Fallon, Ellen Degeneres and Seth Meyers
... is to be considered as one single noun, and then to consider them "combined" means to sum up (presumably) their comedic talent.
To not include "combined" would simply mean that Conan is "so much funnier" than each individual in the list.