After playing around I found that Hot Toys ankle pegs seem to fit the E&S bodies. That's what gets me about all the newer figures....the ankle pegs go directly into the boots so none of the nudes ever come with feet. So if you're looking to use them that way you have to figure out how to get feet on them. A few times I have actually ground down the boots or shoes into a rough foot-like shape and used them that way. I bought a bunch of the knockoff feet on ebay and they work on some bodies but like you said the E&S ankle pegs seem to need a bigger than average ball.
Simple answer is no. But there are parts that might work, and others that won't at all. Ankles and heads between the brands... I am going to say no. That being said, I haven't tried to swap between those brands. It's also hard to say because there are already at least 3 versions of the Soldier Story bodies, and 2 of E&S, more if you count the sized bodies. There are a bigger build and a more slim build, but they use the same joints on the newer bodies.
I think only the wrist pegs might work, and even then, I would check the diameter of the pegs between different versions of the bodies. They have both seemed to use the passed-on long style pegs like the old BBI G3.5 pegs. But the length and diameter may have evolved a bit too. The rest most likely will not.
E&S is moving into another newer generation of body very soon with the 3.0 body, and that will change compatibility again. Early E&S body stuff doesn't even fit with the most current E&S body parts, and with the bodies about to change again, we're about to get another version of stuff that may or may not fit. I had some nice boots from an early E&S figure, and they don't fit the ankle pegs on the current new body at all. Totally different sizes. Same with the heads and necks. And it would require some major surgery at the ankles to get them to work(cutting and re-gluing, or very careful filing and material removal). And E&S loves those molded beets, for a nice sharp look. But they have no flex in posing, so your wanting to put feet on them makes sense to me.
It might work better with newer Soldier Story bodies, but older SS bodies had really small ankle pegs, and proper feet, and those most likely will not fit the ankles on the E&S bodies, at least without some modification. Again, some cutting and gluing may be required with the pegs to get them to cross over.
Adapting those other brands like HT, or any of the clones available, might be an option. I have heard of some folks making 3D printed joints to adapt, other parts. Blackopstoys is making printed extenders and other joints. You might even be able to find printed feet somewhere too.
OSW member Delusional makes several different ankle and neck adapters that are all on Shapeways but nothing for E&S. I got Hot Toys ankle pegs to work for this particular E&S body but that's very interesting to know that they have different body versions whose parts are not interchangeable.
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