Power ups, son.
I don’t see who the SHs can tag team with against the F5. After Marco, the M3, Yamato, and the Supernovas (none of which will tag team a F5 since they’re all stronger than one to begin with), the next best characters are complete nobodies or extremely weak characters who, unlike the SHs, don’t have any growth potential. Sure Shinobu could help Nami vs Ulti, but how useful can she really be? Who would even tag team the rest anyways? Is one of those Yakuza bosses that gets treated like background fodder gonna tag team Who’s Who with Brook?