Sounds good. They should gain natural armor to account for the plate-like chitin. AC 0 translates to AC 20 by the conversion guide.
Modifying the bat swarm stats...
Diminutive Fey (Swarm)
Hit Dice: 3d6 (10 hp)
Initiative: +2
Speed: 15 ft. (3 squares), fly 60 ft. (perfect)
Armor Class: 20 (+4 size, +2 Dex, +4 natural), touch 16, flat-footed 18
Base Attack/Grapple: +1/-
Attack: Swarm (1d6)
Full Attack: Swarm (1d6)
Space/Reach: 10 ft./0 ft.
Special Attacks: Distraction, wounding
Special Qualities: Immune to weapon damage, low-light vision, swarm traits
Saves: Fort +1, Ref +5, Will +5
Abilities: Str 3, Dex 15, Con 10, Int 9, Wis 14, Cha 4
Skills: 30
Feats: 2
Environment: x
Organization: x
Challenge Rating: 2
Treasure: None
Alignment: Usually neutral evil?
Advancement: None
Level Adjustment: -
Int was given (9). We should probably boost Cha to at least 9 as well.
I modified the speed by the conversion guidelines.