Nids are 40K not LOTR.. Some nids have Fleet of Foot/Claw/Hoof so they can move additionally in the shooting phase and some can charge further than normal in the assualt phase so to answere your question.. no not all nids move just 6 inches.
Ok, in 40k movement is based largely upon unit type.
Infantry and Monstrous Creatures move 6" and charge 6"
Beasts move 6", charge 12" and have the Fleet rule (meaning if they don;t shoot they can move an extra 1d6" in the shooting phase)
Jump Infantry move 12" and charge 6"
Tyrants and Fexes are Monstrous Creatures (Unless with wings)
Rippers, Guards, Warriors, Lictors, Termagaunts, Genestealers, Raveners, Zoanthrope, Biovore are all Infantry
Hormagaunts, Leaping Warriors and Leaping Gaunts are Beasts
Gargoyles, Winged Warriors, Winged Gaunts and Winged Tyrants are Jump Infantry (Tyrants are still MCs, but move as JI)
Anything with Fleet (Fleet of Claw) can decide to move an extra 1d6" in the shooting phase instead of firing.
This info can be found in the main rulebook.
And this really should be in the rules forum...
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