I've seen a few of these and I wanted to take a stab at one of my own:
Nucleids
Legends about about the origins of the rare nucleids—a race of sentient oozes that hold humanoid form. Some say the first nucleids was an use that digested a victim slowly enough that its brain became integrated before it dissolved. Others claim they are just the latest crazy experiment by the newest mad wizard. Regardless of their origins, the hardy and flexibile nucleids are here to stay.
Nucleids have a body that consists mostly of acidic ooze held together in humanoid shape by a durable, translucent membrane. Suspended inside their bodies are their slightly less translucent internal organs.
[spoiler=Traits]
+4 Constitution, -2 Intelligence, -2 Wisdom, -2 Charisma: Nucleids are incredibly durable, but not very bright.
Evolved Ooze: The nucleids creature type is Humanoid (nucleid), but they count as both humans and oozes for any effect related to race.
Variable Anatomy: Nucleids have complete control over their anatomy, both internal and external. Nucleid have a 25% chance to ignore critical hits against them. In addition, a nucleid wearing no armor recieves a +10 racial bonus on escape artist checks. In light armor, they recieve only a +5 bonus, in medum armor they recieve only a +2 bonus. Nucleids wearing heavy armor to not gain a bonus on Escape Artist checks.
Ooze Resistance: Nucleids gain acid resistance 5, DR 5/piercing or slashing, and receive a +2 racial bonus on saving throws against poison, sleep, paralysis, polymorph, and stunning.
Acid Body: When making an unarmed strike, natural attack, or damaging an opponent in a grapple, a nucleid can choose to deal acid damage instead of the normal type of damage for that attack. If they do, they use their Constitution modifier on the damage roll instead of strength.
Does damage reduction work against their unarmed strikes or just acid resistance?
If they're choosing to deal acid damage, I would say no. They basically get a choice to deal either acid or standard bludgeoning (doign a standard punch or slinging acid on them, basically)
At later levels, it seems like this race would be good at drunken master monk and not much else, I worry. Two of the racial abilities really only benefit unarmored and unarmed characters and a minus 2 to all metal stats is rough.
Meanwhile, the damage resistances at level 1 would essentially shut down any enemy with a bludgeoning weapon (being probably too good). That critical resistance is also powerful. Is there another race with out level adjustment that gets that?
I would consider making the DR lower and have it scale up with feats (like maybe get it upt to 15 or twenty over the course of 18 levels and three feats) and just let the critical hit resistance start at like 5 or 10 and have it scale with level too.
Why don't you let variable anatomy benefit armor wearers too?
How about let them dawn and remove any piece of armor as a full round action? Then when they are trying to escape from a grapple, they can get a +12 on the check if they are willing to abandon their armor (for light armor). Then scale that up to +15 for medium and +20 for heavy.
Dotting for interest.
Personally, I would probably remove the DR & replace it with something else.