I’m not 100% sure what you mean, but I think this just involves creating a regular lesswrong link the same way you would in any other lesswrong post, comment or tag.
quickly check if a wiki-tag already exists
Ah, this isn’t currently possible on the create-tag page, but is possible when adding tags to an individual post (while on a post page, you can click “add tag”. You can type words into the search box, and see a dropdown of tags sharing those words. If you end up not finding an appropriate tag, you can then click the ‘create tag’ button (which doesn’t automatically autofill the tag name into the form, but the words you typed will still be very accessible and it’ll be reasonably easy to create the tag there)
Unfortunately this isn’t that useful to creating tags that you don’t want to apply to a particular post. I agree we should add a similar dropdown the create-tag page.
I don’t think we currently do handle references any particular way. If a reference is relevant, add a link to it in the description. (I think it works best if you include a paragraph explaining why it’s relevant, but simply adding a link at the bottom is better than not-that)