What are the equipment restrictions for NPC/Hirelings in Diablo 2?

I am playing my way through Nightmare Diablo 2 on Battle.net closed servers and because of the swarming that's occurring I've decided to bite the bullet and get a hireling to come help out.

My act 1 hireling seemed to be restricted to bow weapons. My act 2 defensive hireling comes with a spiffy cold effect but seeems to be restricted to pole arms, without a shield regardless of the class of pole arm (ie 1 handed pole arms should allow a shield, but don't).

What are the restrictions on hirelings and what other bonuses might I discover?

Solution 1:

Those Hirelings are also known as Mercenaries. As listed on Diablowiki.net only the weapon and off-hand slots have restrictions as followed:

Rouge Scouts (Act I)

• Only Bows (NO Crossbows or Amazon only Bows)
• can use Cold Arrow or Fire Arrow in addition to Inner Sight

Desert Mercenaries (Act II)

• Spears, Javelins and Polearms (no Amazon only equipment)
• use Jab Attack and have a Aura dependent on type and difficulty they were bought in:

. |Normal/Hell|Nightmare | Combat |Prayer |Thorns | Offense|Blessed Aim|Might | Defense|Defiance |Holy Freeze|

Iron Wolves (Act III)

• Shield and Sword

• skills depend on type:

  Inferno or Fireball for Fire
  Glacial Spike, Ice Blast, Frozen Armor for Cold
  Charged Bolt and Lightning Bolt for Lightning

• they will not hit much with the sword - look for caster-bonus on it (for example, Spirit runeword)!

Barbarians (Act V)

• Barbarian class-specific items, one sword (no dual-wielding)
• use Bash and Stun

Additionally to the type restrictions, mercenaries follow the items requirements for Strength, Dexterity and Level. Back when I played Diablo II, act II Mercenaries were the most common choice due to their buff-auras.

Solution 2:

Mercenaries follow the following sets of equipment rules:

Act 1 - Rogue Scouts

Armor, Helms, Bows (NOT crossbows or Amazon specific bows)

Act 2 - Desert Mercenaries

Armor, Helms, Spears or Polearms (NOT Amazon specific spears)

Act 3 - Ironwolves

Armor, Helms, Shields, 1 handed swords

Note that the Ironwolves do not actually physically attack with the sword, so finding one with good casting stats for whatever element you choose is is a better idea than finding one with good weapon damage.

Act 5 - Barbarians

Armor, Helms, Barbarian class specific helms, Swords (both 2h and 1h variety, though they cannot dual wield)

Also note that Mercenaries follow STR/DEX requirements for items as well as level, though generally speaking this is not usually an issue.