Improved Monster classes were created from the start as a replacement for the LA system, not something to be used with the LA system.
As opposed to the largely-arbitrarily-segregated Enhanced Monster Classes, which instead replace LA by having an actual playable race version and a class to go with it. That died due to too-harsh restrictions, didn't it?
Enhanced Monster Classes arose out of a large disagreement about Oslecamo's project and so said disagree-er went off to make their own project. That died mostly due to a lack of participation in the project because most people didn't have issues with Oslecamo's project.
That's why they're segregated.
I can see that being the case. But the idea of keeping actual race picks doesn't seem to be that unusual to me.
Edit: Also, at least Oslecamo uses his monster classes for monsters and not just PCs and thus all of his monsters that use the classes use the rules for the classes. It's not two tables, one for PCs and one for Monsters. It's just how this project is structured.
Thank god for that, exceptions-based mechanics lead to hell for actually keeping track of everything. As for not using monster classes for NPCs/monsters... wut. What's the point of making a class for monsters if you don't use it for actually making monsters of a single sort scale easily?+
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I just noticed that there's not a Hydra monster class. Some things about the Hydra that are good for streamlining class creation:
1. The HD is always equal to number of heads, while CR for the normal one is one lower than HD.
2. Cryo and Pyro add 2 CR flatly, coming with Energy Immunity to one type and Vulnerability to another, canceling out one Hydra head weakness. They also grant a Breath Weapon.
3. The Fast Healing inherently scales with number of heads.
Some adjustments to the existing mechanics of the monster to better work with a class:
1. Head HP being 6+Con, rather than fraction of total. This is to keep it to one HD per head when making HD slightly independent of heads.
2. Fast Healing needs to be changed from 10+heads to something scaling slower. Maybe Con+HD, instead, to ground it better.
3. Cryo/Pyro breath weapon needs to be altered. 1d4 rounds cooldown
for each head is too much AoE power, and the 3d6 damage needs scaling.
4. Each head can use it's bite during a Standard Action attack without restriction. This needs changed. Penalties for extra head attacks, like TWF.
Some things to make it actually a monster class:
1. Base going to 12 heads/Colossal size, whichever hits first, with PRCs built for Hydra use that do not require it to fill remaining pre-Epic progression.
2. Each level(or 2), make a pick from a list. List includes Cryo/Pryo abilities in adjusted form, extra heads, additional/unlock of Fast Healing, etc...
3. +1 Con every one or two levels, replacing the typical Con to NA with the Fast Healing, sheer HP and making reach a massive hazard zone.
4. Start at Small size, get Growth every other level after first. This means that it's Medium at 3rd level, Large at 5th, Huge at 7th and so on.
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Tempted to work on it now, but it's 1 AM where I'm at, so I need to go to sleep.