What's the difference between какой and как when they are all used for exclamation?
(note: I know that they can also be used as interrogatives but I am just confused about how these two words differ when they are used for EXCLAMATION)
Same as always: какой is used with adjectives and как is used with adverbs or predicate words (including short adjectives, if you classify "хорошо" or "рад" as such).
Basically, какой has an adjectival ending and is used with full-form adjectives (which also have adjectival endings):
- Какой хороший мальчик!
- Какая красивая картина!
- Какие большие дома!
- Как я рада!
- Как хорошо!
- Как вовремя вы пришли!
- Как быстро она бегает!
Note that it won't be used with adjectives/adverbs that are not commonly understood to have different degrees (e.g., "отлично"). Such words will not take «так» as well.
КАКОЙ is sometimes used with a noun without any adjective. Similarly, КАК is sometimes used as an adverbial "so much, so well" when describing a verb:
- Только подумай, какой случай! ~ Just think, what an opportunity it is!
- Какая красавица! ~ What a beauty!
- Как ты рисуешь! ~ You draw so well!
- Как я вас всех люблю! ~ I love you all so much!
In such use it is implied that you mean the enormous extent of a positive quality. Not the only way to say this, of course, but such structures are founf in Russian.