Yeah, there's a confusion here between self-interest and selfishness. I wrote two pretty thorough articles on how this confusion shows up in economics (links below).
I agree that self-interest is compatible with helping others (as long as doing so is in your interest, as it likely is unless you're wholly self-sufficient). In that case, indeed, self-interest needn't entail that you exclude the interests of others from your practical calculations.
But these are quite slippery grounds, as I show in those articles. For instance, take that quotation from Capitalism: The Unknown Ideal. My questions are simple: What's meant by the word "own" in "the right to exist for his own sake," and is that word consistent with the clause about how we shouldn't expect others to sacrifice themselves for us? If you're existing for your own sake, aren't you implicitly at least weighing your interests as being more important than those of others, especially as compared to the interests of those who don't affect your business?
Thus, even if you don't intend for others to be your slaves, your single-minded devotion to benefitting your interests may result in severe and even counterproductive social asymmetries. In a race, someone wins and everyone else loses. How is a society supposed to function if it's treated as a fair race? What becomes of the social contract if the majority inevitably turn out to be losers by comparison with the few winners?
I'll grant you this, though: I suspect that Rand was personally less one-sided than some of her followers since she apparently repudiated libertarianism.
