Why rewrite your stat blocks for every type of angel and demon? Instead, let's just write up two creatures and let the HD denote their powers instead.
(HD X, can transform into creatures with HD = X-1 or less, protects number of noncombatants equal to X from harm, can reflect damage equal to X, appears as pure white animals and creatures)
Angels are the protectors of good and the innocent, and never appear in the mortal realm as their “true” form. Instead, angels take the form of pure white animals and other creatures, with all their abilities. They can also take human form, but only do so in times of great strife. The maximum HD of a creature an Angel can turn into is equal to their HD -1.
Simple HD 1 angels can only transform into HD 0 things like a white mouse, white dragonflies, or a cute snow bunny. HD 2 Angels could turn into an HD 1 white cat or dog, or perhaps a human child. HD 3 Angels could turn into a white wolf HD 2, or perhaps a great white stag, and so on. White tigers and elephants could be used as combat forms, but some angels prefer to stay as a dove or swan and use their powers instead of fighting directly. Extremely powerful Archangels could turn into a holy white dragon, with all of its fury and strength.
Angels can protect nearby people from harm, usually children or innocent bystanders. The protection can be subtle or obvious. For example, an HD 6 Angel could guide a small group of people through an orc army- the orcs would simply ignore them. This protection could also appear as a glowing shield, such as to protect people from a dragon's fire. Angels can also cast Cleric or Sage spells at a character level equal to their HD.
Angels prefer not to attack unless in a combat form. If in a harmless or invisible “spirit” form, angels can instead reflect damage back to black hearted people instead. This reflection also stops the damage in the first place; so a 2 HD angel could turn back a glancing 2 damage arrow. If the amount of damage is over the amount of HD the angel possesses, then they cannot reflect the damage.
Demons are the evilest creatures in existence, unlike the peaceful dead or guardians of the underworld, demons have no respect for the natural order and simply wish the destruction of everything good, innocent, and pure. Once brought into the world, demons will not willingly leave it and have a single horrifying form that is usually something twisted and toxic. Most demons appear as things like skinless dogs that walk on two legs, or obese black-skinned men that walk on all fours, and so on.
Demons are extremely strong fighters, and their horrible fury and demonic aura strikes fear into the hearts of mortals. Demons get +1 to hit and AC per HD from their extreme physical toughness and infernal strength. When first summoned Demons could also exchange some of their HD combat bonuses for special abilities, with a minor ability costing 1 HD and a major ability costing 2 HD. For example, a 4 HD demon may only have +2 to hit/AC but can fly, or 3 HD demon could have +2 to hit/AC but take half damage from frost, etc. Additionally, half of their HD is applied as a negative to all morale checks rolled by hirelings and soldiers that fight it.
Demons grant dark powers to its followers and cult. For this, they can give out boons equal to HD Each point of a boon is roughly equal to +1 to a stat modifier or mimics the effect of a first level buffing spell with an unlimited duration. Every Demon also has a glowing sigil always on or above them unless they are hiding their identity. This sigil is a bit like their name, and cultists must brand themselves with a demon's sigil or use items engraved or painted with the sigil in order to gain a demon's pact and their boons.
Demons also teach the evil rites of their kind to mortals; including blood magic. Most of these rites are things like evil satanic chants and new methods to torture and kill innocent virgin girls, usually to summon more demons into the world. Rites also include new evil ritual magic spells, such as how to permanently sicken a well or make all the goats on your rival's farm lactate blood instead of milk and so on. Each demon knows a number of these rites equal to their HD, and learning a total of 13 new rites will boost a cultist's blood magic skill by one, or caster level by one for dark spells depending on your system of choice.