Just to be clear: IndieWeb does not require any other WordPress plugins.
That said, it runs alongside Micropub just fine, and augments it where possible.
Should you opt to run both IndieBlocks and the Webmention plugin, you’ll likely want to disable IndieBlocks’ Webmention functionality to avoid possible conflicts.
For outgoing webmentions to work as intended, both your “theme” and actual posts should have the right microformats added to them.
For “in-post” microformats, there’s the “Context” block. And, if your site sports a “block theme,” IndieBlocks can be set up to add microformats in just the right places.
While you can use a “classic” theme, it’d have to be microformats2-compatible. (Unfortunately, not many themes are.) Or, you could try creating a child theme in order to add them yourself.