Skull Skull wins

Assuming it's a a six member free-for-all, I would assume that Skull Slicer's extra arms would allow him to parry attacks from multiple foes at once, giving him an advantage in that respect. But then again, considering how we've seen Skull Grinder defeat all six Toa with a single blow, it's doubtful the other Skull Villains would stand a chance against him...

Skull Scorpio, the underdog will take them all down!

Skull Slicer will just parry himself, then get tangled up in his chain and fall over.

Skull Krata

This Skhull!  Okay, if he's not in the running then this is how I'd see the fight going. 

Skull Slicer appears to have an edge with four arms to fight multiple opponents, but I can see him pulling one of these and getting knocked out in the first two seconds. 

Skull Warrior loses automatically for being a Stormtrooper wannabe. 

Skull Basher is strong and knocks out one of the insects; let's say LoSS just because of his performance against the Toa. 

It would then come down to Basher, Scorpio, and Grinder, but then seeing as Skull Grinder kind of, you know, brought the Skull creatures back to life in the first place then he could probably turn them back into bones winning the fight instantly.

bonesiii.

Nah, Skull Grinder wins all.

A battle between their sets...

Skull Grinder is out, he can't lift his weapon

Skull Slicer get tangled up in his chain, resulting in his failiure

Skull Scorpio can't move.

LoSS Loses.

Skull Basher's Axes get Stuck on his Horns,

Skull warrior wins by Default.

Animation wise

Skull Grinder Swings Staff, All of them Shatter.