Most of the sites on the interwiki map should be wikis; however, there are present some useful prefixes that don't meet this criteria, and it would be a shame to remove them (google, cache, sourceforge, and possibly a few others). Beyond that, though, I think you're on the right track.
David Gerard wrote:
"The InterWiki Map exists to allow a more efficient syntax for linking between wikis, and thus promote the cooperation and proliferation of wikis providing free information on the internet. Sites considered for inclusion should probably (1) provide clear and relevant usefulness to the Wikimedia projects (2) be trusted not to facilitate spam links (3) be free content (under a Commons-acceptable license) (4) be a wiki (5) be reasonably developed."