There is no inherent problem with this, although the flavor text often doesn't fit with the Dark Sun setting. My goal is to list the various monsters that "don't fit" and rewrite their flavor text to make them fit.
Frequency: Very Rare
Activity Cycle: Any
Intelligence: Genius (17-18)
Alignment: Lawful Evil
No. Appearing: 1-4
Armor Class: -5
Movement: 15, Fl 24 (C)
Hit Dice: 13
No. of Attacks: 6
Damage/ Attack: 1d4/1d4/1d6/1d6/2d6/2d4 or weapon +6 (Strength Bonus)
Special Attack: Fear, poison, tail constriction
Special Defense: Regeneration, +3 weapons to hit
Magic Resistance: 50%
Size: L (12' tall)
Morale: Fearless (19-20)
XP Value: 21,000 (19,000 if non-magical)
The most terrible baatezu, pit fiends are giant winged humanoids, gargoylish in appearance, with huge bat-wings that can wrap around their body in defense, large fangs that drip with vile, green liquid, and hulking red, scaly bodies that burst into flame when thev are angered or excited.
No baatezu is more terrifying in combat than a pit fiend. The pit fiend uses its 22 Strength (+6 damage adjustment) to attack six times per round, dividing its attacks among up to six different opponents if necessary. It can attack with two hard, scaly wing buffets (1d4 points of damage each), powerful claws (1d6 points), and a bite (2d6 points and poison; save vs. poison or die in 1d4 rounds). The bite also infects the victim with a disease, whether or not he saves against the poison. Pit fiends can also attack with their tail every round (2d4 points of damage). The tail can then hold and constrict the victim for 2d4 points of damage per round until the victim makes a successful Strength check to break free. Pit fiends also carry jagged-toothed clubs which inflict 1d6+1 points of damage; this replaces one claw attack.
When pit fiends arrive on Athas, they are cut off from their magical power sources. Because they are cut off from their divine and arcane power sources they cannot use any of their spells or spell-like abilities until they learn to draw energy as other arcane spellcasters. Pit Fiends only learn how to defile.
Once per round, pit fiends may automatically gate in two lesser baatezu or one greater baatezu. Any baatezu who is gated into Athas must pass through the Gray and have a chance of getting lost. Any baatezu attempting to leave Athas also have a chance of getting lost in the Gray.
Pit fiends regenerate 2 hit points per round. They radiate a powerful fear aura in a 20-foot radius (save versus rod, staff, or wand at a -3 penalty or flee in panic for 1d10 rounds).
Once they learn to draw energy, Pit Fiends gain all the spell-casting abilities of the baatezu, in addition, a pit fiend can use one of the following spell-like powers once per round: detect magic, detect invisibility, fireball, hold person, improved invisibility, polymorph self, produce flame, pyrotechnics, and wall of fire, Once per year, a pit fiend can cast a wish spell. Once per day, a pit fiend can use a symbol of pain; the victim must save vs. rod, staff, or wand or suffer a -4 penalty on attack rolls and a -2 penalty to Dexterity for 2d10 rounds. Even without the ability to draw energy, pit fiends keep their immunities and damage reductions.
Pit fiends are the lords of their home realm, with the great power and station there. They despise being trapped on Athas and are always seeking a way to escape. They learn defiling magic as soon as possible when entering Athas.
Pit Fiends may gather cults of worshipers, dedicated to their power, but the fiend's end goal is always escape. Their greatest desire is to return to the Blood War. Pit Fiends will enslave any other baatezu they come across into their service and will seek to destroy any tanar'ri they find.
The baatezu are not native to Athas and have no positive ecological impact. Baatezu do not reproduce.