Well, since pass is a shell script, and thus has no compilation, you don't need pwgen, and then you can just use "some-generator-program | pass insert blah" as you mentioned. For me, though pass generate Cheese 20 is a lot easier to remember than having to think about the options to pwgen. Pwgen, by default, makes passwords that are easy to remember, and there's some flag you have to hit to make them "truely random". I can't ever remember what this flag is.

With xclip, it's actually a bit more nuanced. You want this to be internal because I have some logic for removing the password from the clipboard after 45 seconds and putting the old clipboard contents back (if nothing else has replaced it in the meanwhile).