Advancing psionic monsters

[Evilhalfling]

Okay, so I am advancing a psionic monster, an Aboleth.
I am assuming its manifester level inreases with its HD, which is very powerful due to the automatic scaling for psionic powers.

Can I use a feat to take Expanded Knowledge to add more abilities to the list ?
what about taking levels in prestige classes - such as Thrallheard? ( a natural fit of aboleth)
does this increase the manifester level of its natural psionic abilities? or give actual psion levels.

 

[werk]

Srd

http://www.d20srd.org/srd/improvingMonsters.htm#tableCreatureImprovementbyType

Aboleth's are aberrations I think.

http://www.d20srd.org/srd/monsters/aboleth.htm

I thought that monsters that list advancement by HD could not advance in classes, but that could be old rules.

I'd just raise him based on the HD advancement and use the HD to determine skills, feats, DC.

 

[Patryn of Elvenshae]

I thought that monsters that list advancement by HD could not advance in classes, but that could be old rules.

Not quite. Anything with an Int of 3 or greater, generally, can take classes. Some creatures have the ability, in addition, to advance by hit dice. Some creatures, however, cannot advance by hit dice at all; those have an advancement line of "-" or "By character class"

Note that, generally, advancing something's hit dice won't increase its caster / manifester level.

For instance, take the aboleth. In the MM / SRD, it's presented twice: once as a standard aboleth (advancement 8-16 HD (Huge), 17-24 HD (Gargantuan)) and once as an Aboleth Mage (Aboleth + Wizard 10, advancement by character class).

It's effective caster level for its psionic abilities is 16th. It remains at 16th even should you advance it to 24 hit dice.

Some creatures, however, cast spell "as a 6th-level sorceror." In such a case, if the creature takes sorceror levels, those stack with its native abilities. For instance, take a particular dragon that casts spells as a 9th-level sorcorer. A Dragon Sorceror 3, then, would cast spells as a 12th-level sorceror.