The rule to compare adjectives is to add -er
to a one-syllable adjective. Yet, the comparative form of just
is not juster, it's more just. The comparative form
of chic is not chic(k)er, it's more chic.
Is there a sub-rule for this kind of discrepancy from the norm?
Rules were made long after the language formed its patterns of usage,
and some of those are undergoing changes even now. The "exceptions"
you point out are remnants of usage that fall outside of the usual patterns.
In the case of "chic," you've noted the spelling quandary
caused by trying to add an English ending to a word imported from French.
We can't even agree on whether to pronounce it like "chick" or "shick"
or "sheek." It's probably an import that is never going to be totally
forced into English patterns.
The case of "just" is probably more related to levels
of formal usage, since this meaning of the word has to do with legal
or political matters. It is seldom encountered in everyday conversation.
Again, this is a word imported into English from French, and thus it
was probably given the word "more" rather than the
ending -er to make sure that listeners understood
clearly what an orator or lawyer was communicating.