Well I think it's stretching it a bit putting these in the "Psionic Monsters" since they don't have any disciplines or sciences. Plenty of AD&D monsters had Psionic Strength and Attack/Defence Modes.
Still, their on this thread so we might as well keep them.
Acheron is far different nowadays than described here. Hades actually makes better sense based on their description. Or we can simply state they are found anywhere along the River Styx.
I was thinking the top layer of Carceri would suit them, since Othrys is filled with swamps and rivers.
Spreading them along the Styx makes the most sense to me, we could say they live in any swamp connected to the river. The Styx flows through all the 1st layers of the lower planes, so that covers a lot of territory.
For ability scores, Int is listed as "very high", which I'm assuming should be "very to high", meaning an Int in the 11-14 range.
Here are the ability scores of some other 14 HD fiends:
Nalfeshnee (H): Str 25, Dex 13, Con 27, Int 22, Wis 22, Cha 20
Nycaloth (L): Str 20, Dex 14, Con 25, Int 13, Wis 10, Cha 16
Gelugon (L): Str 23, Dex 21, Con 23, Int 22, Wis 22, Cha 20
So maybe...
Str 20, Dex 21, Con 25, Int 13, Wis 10, Cha 16?
Hmm, that Dex seems awful high for a blob, I'd rather have 16-17 or so.
Str could do with being higher, since they're grapplers.
Wisdom 10 doesn't feel right for a creature with psychic attacks, I'd rather it were higher so it has a respectable Will saves for a 14HD Outsider.
Maybe
Large Outsider
Hit Dice: 14d8+98 (161 hp)
Str 24, Dex 17, Con 25, Int 13, Wis 16, Cha 16
It will need a +10 natural armour to get AC22 with that Dexterity, although we should probably be aiming for something more like AC30 (+18 natural) for an Outsider of that HD.
As for the alignment, they're created by a Demon Prince but have rebelled and turned Lawful, which is interesting. Giving them a neutral alignment component doesn't sit with me well though.
Maybe give them the Evil and Chaotic subtypes with an alignment of Always evil (often lawful)?