We have learned that kostym is a suit, but can be used for costume also? Like a halloween one?
I would say that 'kostym' is a common mistranslation of 'costume'.
"It's recommended that your wear a 'kostym' to a job interview; but don't put on a 'costume'."
So, we use the '-a' form of an adjective with possessive nouns; but not here? Is 'perfekt' irregular, or is that only when you use the adjective directly next to a possessive? What is 'perfekt' matching exactly?
Swedish uses both attributive and predicative adjectival forms. What this means is that if the adjective comes before the noun, you use the definite form - and if it comes after the noun, you use the indefinite. Think of it as "the silly jester" versus "the jester is silly" - in English, there's no difference in the adjective, but Swedish frequently has different forms for the two.