I think the idea behind the WotC design theory is that combat is combat, but critters with strong inherent abilities must put in the same amount of practice to master their innate abilities as spellcasters do with their arcane or divine studies.
But you could try this- adopt the Arcana Unearthed/Arcana Evolved approach to monster classes.
The system there is very similar to Savage Species, but there is one crucial difference: in AU/AE, a "monster" starts off with some weak-ish initial stats in its LA +0 form, just like Savage Species...but in AU/AE, it doesn't EVER have to take a single level in its monster class.
(For those who don't know or recall, in Savage Species, a monster PC must take all of its available monster levels before taking a PC class level.)
This would let you have a 5th level Succubus with 10 Sorcerer levels, or 7Succ/8Sorc or whatever.
But you could try this- adopt the Arcana Unearthed/Arcana Evolved approach to monster classes.
The system there is very similar to Savage Species, but there is one crucial difference: in AU/AE, a "monster" starts off with some weak-ish initial stats in its LA +0 form, just like Savage Species...but in AU/AE, it doesn't EVER have to take a single level in its monster class.
(For those who don't know or recall, in Savage Species, a monster PC must take all of its available monster levels before taking a PC class level.)
This would let you have a 5th level Succubus with 10 Sorcerer levels, or 7Succ/8Sorc or whatever.