Should I use somebody else's vocabulary of terms or make my own?
Yes.
see also: DontWorryBeCrappy. Perhaps more helpfully, see VocabularyMarket. Use somebody elses rather than making aliases on purpose. It's one thing to make your own and then discover that there's something equivalent out there. It's quite another to willfully clutter the semantic web with aliases; the latter increases the burden on the community of consuming your data, so it's anti-social. (discussion with respect to WWW2004 photo annotation project)
Comment: I suppose an obvious thing is that you should be able to suggest/specify in a specification and associated namespace what terms you would like/want someone to use from other vocabularies...
is cyc too big? Can I use just part of it without buying into all of it?
is cyc's license too scary?
will foaf go poof?
I worry about using RDFS, swap/contact, and the like; I think a HashURI is for referring to a part of a document, so GoodURIs for RDF properties shouldn't use them. What should I do?
I worry about foaf, dublin core, XMP, and the like; I think http URIs without fragment identifiers are for web pages (and things you can POST to), so GoodURIs for RDF properties should use fragment identifiers. What should I do?