Now I know the first reaction is ... "Oh sure suggest changes to benefit your character." While this may be true, I think that you will see that my suggestions will do little to change the strengths but rather make the summons more fitting.
Level 1:
Current - Fiendish Hawk, Animal 2 Outsider 2, 25 HP, 13 AC, DMG Reduction, 1-4+2 dmg, +5 attack
Proposed - Dretch, CR 2, 9 HP, 16 AC, 5/silver, SR 5 (10 is NWN Minimum), immune poison, immune electircity, 20/- cold, fire, acid, spell of darkness and stink cloud 1/ day, 1-4+3 dmg
Advantage - Toss up: Hawk has DMG Red and more HP, Dretch has higher AC and some spells, honestly the elemental resistances aren't going to do much for him with the low HP
Level 2:
Current - Fiendish War Dog, Animal 3 Outsider 3, 38 HP, 13 AC, DMG Reduction, +7 attack, 1-6+2 dmg
Proposed - Quasit, CR 3, 13 HP, 18 AC, 5/silver, SR 5, immune poison, 20/- fire, Regen +2, invisiblity and cause fear 1/ day, 1-3 +poison
Advantage - War Dog, the HP and DMG Reduction out way the AC of the Quasit
Level 3:
Current - Fiendish Wolf, Animal 4 Outsider 4, 59 HP, 15 AC, CHR STR CON DEX INT Increased, DMG Res, DMG Red, immune poison, DMG Res fire, cold, elec, +11/ +6 Attack, 1-4+4 dmg
Proposed - Nightmare, CR 5, 45 HP, 24 AC, alertness, improved initiative, 1-8+4 / 1-4 Fire / 1-8+2 dmg
Advantage: Toss up - This is a toned down version of the current Nightmare and based upon the Monster Manual
Level 4:
Current - Hamatula, Outsider 9, 49 HP, 10 AC, DMG Red, DMG Res, Fire Res, Immune Poison, Spell Res, +12/ +7 attack, 2-8+4/ 2-8+3 dmg
Proposed - Bodak, CR 8, 58 HP, 15 AC, death gaze, 15/silver, 20/- fire and acid, elect immuinty, sunlight weakness, slam+ 1-8+1 dmg
Advantage - Well, first I have a problem with a Hamatula being summoned by a Cleric of Corath, Corath governs Demons not Devils. This version of a Bodak is out of the Monster Manual. But I woud put advantage Bodak
Level 5:
Current - Nightmare, Outsider 6, 88 HP, 21 AC, dmg increased, dmg res, 1-8+4/ 1-8+4/ 1-8+4 dmg, +1-/ +5 attack
Proposed - Bebilith, CR 9, 102 HP, 25 AC, web, poison, dmg red 30/+3, aura, 2d6+9 poison/ 2d4+4 dmg
Advantage - Bebilith, in the current make of the nightmare it is very weak and should be re-evaluated as to its CR, the bebilith is a demon and is keeping with the governs of Corath.
Level 6:
NO CHANGE - Doom Knight
Level 7:
Current - Gelugon, 114 HP, 29 AC, dmg increased, attack increased, regen, dmg red, dmg rest, fire resist, immune poison, spell resistance, stun, +22/ +17/ +12 attack, 2-8+6 1d6 acid +4 pierce dmg
Proposed - Vrock, 60 HP, 25 AC, spores (1d8 dmg + 1d2 each round after), stunning screech, immune poison, 20/- fire, cold, acid, immune elect, 1-8+4/ 1-6+2/ 1-4+2 dmg
Advantage - Gelugon, but here again the gelugon is a devil not a demon.
Level 8:
NO CHANGE - Glabrezu
Level 9:
NO CHANGE - Maralith
Now I know that a lot of this is dependant upon balancing issues, and I know to becareful what you wish for. My biggest thing in this whole post is getting rid of devils and replacing them with demons. Comments, Questions, Disagreements? Weither any thing changes as a result of this thread or not matters not. The GM team we have does an excellent job here, and this thread is in no way complaining, mere suggestions.