If I'm building a Tau deck with Shadowsun, can I include other Warlord's signature cards in the deck?

No. Here's what the Learn to Play guide says about deckbuilding (it's at the very end):

Deckbuilding Checklist

When constructing a tournament deck, a player first selects a warlord. His deck then includes:

• The 8 cards from the chosen warlord’s signature squad (mandatory).

• Up to 3 copies each of any non-signature card(s) from the warlord’s faction.

• Up to 3 copies each of any non-signature, non-loyal card(s) from one (and only one) neighboring faction on the alignment wheel.

• Up to 3 copies each of any neutral card(s).

• A minimum of 50 total cards, not including the warlord itself

Boom, thanks.

Note that Tyranids and Necrons operate under slightly different rules, but the changes concern only the 3rd and 4th point:

• Tyranids cannot include cards from other factions and cannot include neutral units.

• Necrons can include non-signature, non-loyal (since this is a bit long, the term "common" was introduced in the Planetfall cycle) units from any number of factions on the wheel (so no Tyranids), and any neutral cards.

Also note that individual warlords can modify these rules (those changes continue to be valid when a warlord is bloodied, legal game play cannot make a deck illegal). For now, 2 warlords (Gorzod and Commander Starblaze from the Planetfall cycle) modify the 3rd point.