Am I correct in thinking that, while I can have multiple blogs per micro.blog account, each micro.blog account can have only one “Mastodon-compatable Username”?
If so, are post from all blogs published to mastodon as toots? Or is there a way to configure/filter this?
Yes, you’re correct, one “Mastodon-compatible username” per account.
People following you from Mastodon or other ActivityPub-compatible software will see the post from your Micro.blog timeline: http://micro.blog/jemmons. You control what appears on there by adding and removing feeds on the Edit Feeds and Cross-posting page.
As long as there’s a web feed (like RSS or JSON Feed), it can go on the timeline. Posts from all of your blogs, none of them, some specific categories from a couple of your blogs, someone else’s blog, or updates from a YouTube channel. Anything goes.