Hips braucht keine SA, weil diese SU erlaubt lediglich eine Anwendung von Stealth in sonst nicht möglichen Optionen. Es ist keine besondere Aktion an sich (und braucht daher keine gesonderte Aktion auf).
Action: Usually none. Normally, you make a Stealth check as part of movement, so it doesn't take a separate action. However, using Stealth immediately after a ranged attack (see Sniping, above) is a move action.
Linkaber mit dem Schatten bin ich mir trotzdem noch nicht sicher, weil - außer in der Definition von HiPs - findet sich nirgendwo in PF oder 3.5 eine Definition von Schatten. Sprich: Schatten sind auf die Auslegung des DM angewiesen.
Dafür, dass man die Nähe zu anderen Kreaturen nutzen kann um sich mit HiPs zu verstecken, spricht aber, dass diese bei Ranged Attacks Cover geben:
Link: Cover
To determine whether your target has cover from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target's square passes through a square or border that blocks line of effect or provides cover, or through a square occupied by a creature, the target has cover (+4 to AC).
When making a melee attack against an adjacent target, your target has cover if any line from any corner of your square to the target's square goes through a wall (including a low wall). When making a melee attack against a target that isn't adjacent to you (such as with a reach weapon), use the rules for determining cover from ranged attacks.
Low Obstacles and Cover
A low obstacle (such as a wall no higher than half your height) provides cover, but only to creatures within 30 feet (6 squares) of it. The attacker can ignore the cover if he's closer to the obstacle than his target.
Cover and Attacks of Opportunity
You can't execute an attack of opportunity against an opponent with cover relative to you.
Cover and Reflex Saves
Cover grants you a +2 bonus on Reflex saves against attacks that originate or burst out from a point on the other side of the cover from you. Note that spread effects can extend around corners and thus negate this cover bonus.
Cover and Stealth Checks
You can use cover to make a Stealth check. Without cover, you usually need concealment (see below) to make a Stealth check.
Soft Cover
Creatures, even your enemies, can provide you with cover against ranged attacks, giving you a +4 bonus to AC. However, such soft cover provides no bonus on Reflex saves, nor does soft cover allow you to make a Stealth check.
Big Creatures and Cover
Any creature with a space larger than 5 feet (1 square) determines cover against melee attacks slightly differently than smaller creatures do. Such a creature can choose any square that it occupies to determine if an opponent has cover against its melee attacks. Similarly, when making a melee attack against such a creature, you can pick any of the squares it occupies to determine if it has cover against you.
Partial Cover
If a creature has cover, but more than half the creature is visible, its cover bonus is reduced to a +2 to AC and a +1 bonus on Reflex saving throws. This partial cover is subject to the GM's discretion.
Total Cover
If you don't have line of effect to your target (that is, you cannot draw any line from your square to your target's square without crossing a solid barrier), he is considered to have total cover from you. You can't make an attack against a target that has total cover.
Improved Cover
In some cases, such as attacking a target hiding behind an arrowslit, cover may provide a greater bonus to AC and Reflex saves. In such situations, the normal cover bonuses to AC and Reflex saves can be doubled (to +8 and +4, respectively). A creature with this improved cover effectively gains improved evasion against any attack to which the Reflex save bonus applies. Furthermore, improved cover provides a +10 bonus on Stealth checks.
Da scheint einfach bei der Definition von HiPs viel zu fahrlässig vorgegangen worden zu sein... vielleicht könntest du das Hausregeln. Da eigentlich alles, was im Spiel und Regelwerk Concealment oder Cover gibt, RAW einen Schatten wirft oder als Schatten ausgelegt werden kann:
Vorschlag für Hausregel:HiPs erlaubt es sich in 10' Entfernung von einem Feld, welches Konditionen für Cover oder Concealment gibt, Stealthchecks zu machen.
Das Problem mit der "gerade Linie" bei Kreaturen als Cover wird durch die Fähigkeit ja auch unterbunden. Und der HiPs nebensatz: "She cannot, however, hide in her own shadow." macht dann auch wieder Sinn - man kann sich ja nicht selbst Cover geben.
Aber das ist deine Entscheidung! Nur das der Stealth Check mit Nutzung von HiPs plötzlich ne Standard Aktion bräuchte, damit stimme ich nicht überein.