More than anything, SCDP functions as an archetype for the Madison Avenue of the 1960s rather than as something modeled (even in part) after a real firm. Certainly, some of the work Draper Daniels did could tie SCDP into various firms that Daniels worked for, most notably the Leo Burnett Company. But that connection likely serves more to support the notion of Draper as creative genius than anything.
Other connections are possible, too -- I've always thought that Young & Rubicam, an actual agency which is mentioned frequently on the show as a firm in Mad Men's world as well (think of the water bombing scene in the 1st episode of Season 5), has a similar history to Sterling Cooper -- founded in 1923 by two partners, major creative force in the 60s, etc. (This connection might be strengthened by noting that Young and Rubicam is one of the few ad firms still operating in its original Madison Avenue space, a history which would be appealing to Weiner in the arguments he's trying to make about the place of his show in today's culture).
But many other moments in the show make it very apparent that SCDP is really meant to evoke a cultural discourse rather than a historical context. Think of the "It's Toasted" slogan from the pilot ... an actual slogan from Lucky Strike in 1917, not 1960. Such anachronisms point to Weiner's attempts to make SCDP more about "Madison Avenue"-ness rather than anything truly concrete.