HD slot Issues and possible solutionsCase 1: Can't take advantage of monsters that are low level
Problem: low level mooks only stack if they have class levelsThat isn't a problem. You cannot be more ogre than an ogre can be. It doesn't make any sense.
If the ogre was Ogre 3/Barbarian 2, you could steal all those HDs and be the same.
My system allows you to take the 3 levels of Ogre and get HDs from something else as well.
If you ate an Ogre 3, then a Warblade 3, you could mimic a Ogre 3/Warblade 3 if you took their HDs.
A PC that took the 3 levels of Ogre cannot get more Ogre levels, why should the MD do?
Level equivalencies would make hunting even easier than it already is and cheapen out the entire experience.
If you ate an Ogre 3 and a Troll 4, you could be Ogre 3/Troll 3 if you cared, then switch to Ogre 2/Troll 4 the next day. If levels were equivalent, just eating an Ogre would allow you to multiclass every monster in the book. Fluff wise, it doesn't work either since the memories of an ogre are probably quite different than a troll, in example.
Case 2: transitioning is painfulOn the other hand, transitions are actually possible. The current system is all about finding exactly what you want, then finding it again. This one actually allows you to get what you aim for and keep on improving it. If you want to switch to something entirely different, then you can try to find exactly what you want, like the current system. But with this you can actually get something close to what what you want, then steer it closer with other victims.
I agree that you can if you get a new idea in mind, the transition could take long if you take partial HDs instead of waiting to find something something better.
Yet, as I said, increasing the base Pool is a solution. What I think would work best are Mindstealer Drone feats allowing you to get a better pool. Since you eventually get the ability to swap feats around, you could even trade those when you don't need them for a transition.
Better yet, it keeps the spirit of the original creature intact; looking for one big prey at a time or snuffing in large amounts of weak people.
Absorb (Feat), (Aligned Shape), (Extraordinary) and (Supernatural) delayedCase 3: Gimped below level 8I think you misunderstood the way it works. You get class abilities. When feats are a class ability, like with the fighter, you get 'hem. Absorb (Feat) only allows you to trade feats you already have for feats some dude you absorbed got. I think I should even put a clause on Absorb (Feat) to state you cannot trade your feats for feats granted as a class features, like a monk's.
If you want to play casters t'ill you can shuffle feats around, nothing stops a player from getting caster feats just as a caster would. Or combat feats, or any other one. Getting that ability only widens your versatility when you reach it. With what the MD class offers before then, your options are already wide enough to keep you on par with the other classes.
Case 4: Don't get to play monsters with many cool abilitiesI think you misunderstood the way (Extraordinary) and (Supernatural) works.
Those are mainly for monsters that are not already part of Osle's list.
As I said, when you get class abilities, you get 'hem. If a monster class has Growth as a class ability, it isn't the creature's 'base size', its a class ability; you get it.
There is a difference in getting class abilities and something a monster has just for being the monster, and this is when those two Absorb abilities come into the picture.
Case 5: Lots of Paperwork 
In your example, you have it absorb 6 different creatures, so 6 sheets the DM can just hand you over.
Osle's version allows you to keep track of as many people as your HD, so that is 10 sheets.
You keep track of 6 sheets instead of 10. How is that more paperwork?
