This is what you're missing

<ToolCapacityDef>
<defName>ARI_ScorpAttack</defName>
</ToolCapacityDef>

<ManeuverDef>
<defName>ARI_ScorpionSting</defName>
<requiredCapacity>ARI_ScorpAttack</requiredCapacity>
<verb>
<verbClass>Verb_MeleeAttackDamage</verbClass>
<meleeDamageDef>ScorpPoison</meleeDamageDef>
</verb>
<logEntryDef>MeleeAttack</logEntryDef>
<combatLogRulesHit>Maneuver_Bite_MeleeHit</combatLogRulesHit>
<combatLogRulesDeflect>Maneuver_Bite_MeleeDeflect</combatLogRulesDeflect>
<combatLogRulesMiss>Maneuver_Bite_MeleeMiss</combatLogRulesMiss>
<combatLogRulesDodge>Maneuver_Bite_MeleeDodge</combatLogRulesDodge>
</ManeuverDef>

Then replace

<capacities>
<li>ScorpPoison</li>
</capacities>

with

<capacities>
<li>ARI_ScorpAttack</li>
</capacities>

Hope that helps.