Squigs don't fight on their own - they have to be herded into battle by Night Goblins armed with pitchforks, firebrands, whips and various loud drums and horns, not forgetting the truly repellent drone of the 'squigpipe'. A Squig herd unit therefore consists of three different elements:
Squigs.
Night Goblins.
Squig Hoppers.
These may be mixed in different proportions but are always arranged into a normal formation of ranks and files, just like other units.
When deploying a Squig Herd on the tabletop, Night Goblins can never be arranged in front of Squigs. They can be placed in a rank behind Squigs or alongside Squigs, but never in front. Squig Hoppers can go in any rank in front, behind or beside Squigs as you wish.
The unit is considered to have the same Leadership value as the Night Goblins because they control the Squigs (in theory). All Leadership based test are therefore taken against the Night Goblins' Leadership, including any Break tests required.