So far the Kid and Law Pirates really haven't shown anything that proves that they can defeat the BMP, and none of them are strong enough to defeat Smoothie.
Only Law, Kid, and Killer are the only ones that can take on Smoothie, and Oda has already given them their own fights. Also even if the BMP do fight, I feel like the SHs will fight the major members while the Heart Pirates and Kid Pirates will fight the lesser known members.