Rubber is indeed versatile, but hardly indestructible. Actually, almost everything has, to some degree, an elastic (stretching and returning to same size) and plastic (stretching but not returning to same size) range of deforming when stretched... Its just that Rubber is most known for its elasticity and also its ability to insulate against electricity/lightning. I just think that if the breakdown of the body when eating two devil fruits is due to a molecular breakdown or something like that, being rubber probably wont save him. Its still a mystery what kind of body Blackbeard has too which made it possible for him to hold in 2 fruits.
Having read this, I actually went back and watched that episode

and sure enough, you're right... it does look like a greenish mist. I dunno if it's just a special effect they used or it has some kind of meaning. I wouldn't be surprised if it was like that as a hint to Dragon's ability and when it's actually revealed people can go back and say "so that's why it was like that at Loguetown!"
As for plasma, its just another fluid. The only difference between plasma, gas, and liquid is the amount of energy the molecules in the fluid has. When the molecules have so much energy that they start losing electrons and becoming ionized is when a gas becomes a plasma state (if I remember right).