Martha Plimpton is still slender-framed (though, as you say, not anorexic in the popular Hollywood manner). I saw her play Laura in The Glass Menagerie in Chicago a couple years ago.
A while back I suggested Lindsay Crouse for Marian; she's versatile enough to play almost anything, seems about the right age range. She has played glamorous on occasion, but has no vanity about playing plain (and her face isn't a conventionally "pretty" one).
Caruso's a tough one. Nathan Lane is the right idea -- chubby, outrageous but lovable, larger than life -- but he'd need to be put through the 200% or 300% setting on the enlarger first.
But I love the challenge of casting remembered real-life people, just to see how the "equivalents" work from one generation to another. That was the appeal of Chaplin for instance (Kevin Kline as Douglas Fairbans, yes indeed). I always thought Julie Andrews should have had the chance to play Irene Dunne in some biopic too. Now who's Rosa Ponselle and Emmy Destinn?