Beyond the suggestions above I should recommend take in consideration the rules for monsters as players of 3rd edition's Monsters Manual/Dungeon Master Guide and mix it with the 2e DMG guidelines and CHHB rules to adapt powerful races like illithids/alhoons specifically. One more book can be useful for this: Ad&d Monstrous Arcana - the Illithiad.
So, In 3e DMG (page 23) we have a rule about monstrous races HDs that can fit better with a 'illithid/lich' who beggining at lvl 1 makes no sense:
If a monster has 2 or more Hit Dice, it can start with no class levels
(though it can gain them later)
The alhoon should have 9 'monster levels', it could begin casting spells as 9th level and with 8+4 HD, like its statistics from the 'Illithiad' (page 87):
Alhoons normally cast spells as 9th-level mages (Spells: 4/3/3/2/1).
However, they cannot cast spells in the same round in which they
psionically or physically attack. Alhoon: AC 5; MV 12; HD 8+4;
THACO 11; #AT 4; Dmg 1d4; SA psionic powers, spells: SD infravision,
immunities, unturnable; SW sunlight, subject to spells that affect
undead—excepting those detailed under Special Defenses below; MR 90%
Then it should begin to gain levels as mage, receiving d4 HDs, improving THAC0 at wizard rate, etc.
It is a very powerful option. If you consider that the worst disadvantage in Humanoids Handbook is for firbolg (page 29):
Firbolgs must earn double the experience points of the base class to
advance each level (for a fighter, 2nd level at 4,000,3rd at 8,000,
and so on). Firbolg Shamans: Firbolg shamans must earn triple the
experience points of the base clerical class to advance as shamans.
The GM could rule that a alhoon must earn triple or even quadruple xp.
Another rule from CHH that fits is monstrous traits, it could have:
Monstrous Appearance 3: Humanoid appears strange and frightening; -6
penalty to reaction checks.
As a powerful race, It could have a high maximum level as mage, since quadruple or triple xp limitations are the bottle neck here. So, the maximum level in CHH (with the exception of saurials) is 14 (swanmay), it could yet get pre requisite score above 3 points the maximum racial and reach level 17 (table 2 on CHH page 9).
And a +2 Int/-2 Cha modifier is plausible in a 2e campaign.
It is implicit here that the campaign should begin at level 9 to the group of adventurers.