Do we say yaaleh v'yovo or r'tsai or both if one eats today which is Friday rosh chodesh after being mekabel shabbos but before nightfall. Does it make a difference if you finish before or after nightfall.
If you davened Ma'ariv (early, BeTzibur), then Rosh Chodesh has effectively ended and Kedushas Shabbos has taken over, even if it still daylight outside. Therefore, you bench Retseh (for Shabbos) and not Ya'aleh V'Yavo (for R.H.). See Mishna Berura, 424:2, (on the right hand side), also referencing Magen Avraham, there. Saying both might be considered a stira (contradiction).
If all you did was MeKabel Shabbos (by saying Mizmor Shir...) and you right away made Kiddush, with the intention of davening Ma'ariv later (Bizmana), then consult your L.O.R.