They all have their own crew or are already part of one, so why would they join the SH?
And just because Kid and Law are fighting by Ruffy's side right now, doesn't mean they are friends. We all know that Kid is most likely (if the WG is not lying) an evil bastard, who enjoys killing. Ruffy would never be friends with someone like him, and he could turn on Ruffy as soon as the marine is dealt with.
And even though Law seems like a cool guy, he could do the same. Oda could let Law attack Ruffy just because we suspect Kid is more likely to do it.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"