Is it so a good idea or such a good idea?

Such a good idea. So never goes directly before a or an.

Can such go with a plural noun?

Yes, with no article: such good ideas, such nice people.

Grammar, compared

So goes before an adjective or adverb alone. Such goes before a noun, with or without an adjective.

So / Such
So	Such
The film was so boring. (adjective alone)	It was such a boring film. (adjective plus noun)
She speaks so fast.	She's such a fast speaker.
It was so expensive that we didn't buy it.	It was such an expensive bag that we didn't buy it.

So and such compared
Point	So	Such
Goes before	An adjective or adverb alone	A noun, with or without an adjective
Pattern	so + adjective or adverb	such (a/an) + (adjective) + noun
Example	The food was so good.	It was such good food.
With a singular noun	Not used directly	such a/an + noun: such a good idea

Tan covers both uses in Spanish, so English forces a choice: tan rico becomes so good before an adjective alone, but such a good meal before a noun.
Don't say so a good idea. Say such a good idea, or the idea was so good.
Such goes before the article: such a day, not a such day.
So...that and such...that both introduce a result: it was so hot that we stayed in, it was such a hot day that we stayed in.

Is it so a good idea or such a good idea?: Such a good idea. So never goes directly before a or an.
Can such go with a plural noun?: Yes, with no article: such good ideas, such nice people.

Book a trial class. She writes back with times in Buenos Aires time.