Hi Ko-San san
I also find 'SUCH' and 'SO' difficult to use. May I throw my two cents' worth?
I) When the noun is with 'a' (countable in singular form), you can use both of 'SUCH' and 'SO'.
1) I didn't realize that he was such a big fool. [such+a+ADJ+NOUN]
2) I didn't realize that he was so big a fool. [so+ADJ+a+NOUN]
I personally feel the expression (1) more natural/colloquial.
II) When the noun is without 'a' (uncountable or countable in plural form) you can use only 'SUCH'.
3) I didn't realize that they were such big fools.
NOT) I didn't realize that they were so big fools.
paco