What is the reason why one should not say kiddush before Shkia (sunset)?
I believe what you are looking for the following - If one brings in Shabbat early by davenning Minchah after Plag Haminchah, he can make the Kiddush right after Tefilah as long as there is more than a half an hour till Tzeit Hakochavim – the night fall. If night fall is within less than a half a hour – one should wait till night fall, say the three paragraphs of the Shema and only then make Kiddush. If one decides to make Kiddush before the Shema, he can rely on the lenient opinions as long as he will say the Shema later. (Mishnah Brurah 267, 6)