Will the AK G2 double hook trigger groups fit in a Saiga .223?

No

Is the Intrafuse Saiga T6 Stock Set compatible with all Saiga rifles?

The Intrafuse Saiga T6 Stock Set is designed to work with most Saiga rifles and shotguns. This includes .223, 7.62, .308, .410, 20 gauge, and 12 gauge firearms.

There are some rare Saigas on the market with an angled cut on the rear of the receiver, and therefore the Intrafuse Saiga T6 stock will not fit these.