The reason I haven't spent much time with Epicurean physics and epistemology is that I am a firm believer in modern science. I think it might be important to contrast Epicurus' ideas with modern scientific understanding.
Parts of Epicurean physics are certainly outdated, but not all by any means, and none of Epicurean epistemology would really fit the "outdated" parts. I think as you read further into it you will find much that you won't otherwise understand as to how the Epicurean Ethics works.
So now we must extend this "do no harm" to all people.
And this would be example number one. I am no expert on Kant, but the notion of "extending to all people" sounds like a Kantian categorical imperative, which I would say has no parallel at all in Epicurus, which would be quite the opposite in viewpoint. In Epicurus there is no universal justice, but in fact only local and subjective justice, for example as stated in PD33 and PD34.
While I don't share the extreme conclusions of some about "live unknown" there is a very strong strain of "subjectivism" in Epicurean philosophy where you don't expect everyone to agree with you on everything, and so with those who don't (who can't be made your friends because of it, you don't force them and yourself to live we each, you "separate." PD39
And there is no reason that you would tolerate others doing what you believe to be harmful to yourself, even though they don't agree with it's correctness. Such people you don't "reform," you "restrain," "Yet nevertheless some men indulge without limit their avarice, ambition and love of power, lust, gluttony and those other desires, which ill-gotten gains can never diminish but rather must inflame the more; inasmuch that they appear proper subjects for restraint rather than for reformation.
I think as you read more into these physics and epistemology issues you'll begin to resolve some of your questions about what Epicurus was teaching. Note that I am not saying that you will agree with them, but that you'll begin to understand why they don't lead toward universal ethical values for everyone at all times and all places. Yes, I do think that it is possible to generalize that in most cases pleasure is to be chosen and pain to be avoided, but we don't even do that ALWAYS even in our own cases -- sometimes we choose pain in order to bring more pleasure to us later.
Possibly the big question in all this is "Whose pleasure?" I think you'll eventually find that the "greatest pleasure for the greatest number" might be Utilitarian philosophy, but not consistent with Epicurus.