So lemmy.ca and piefed.ca have different feeds altogether when I view them, are they two separate things then?

Separate things, just the same admin I guess. You can follow the same communities from either one.

I have a Mastodon account and I haven’t sorted out any way to post on/read lemmy content with it

On Mastodon, follow the account for this community, just Mastodon uses @ instead of ! so it’s @fediverse@lemmy.world and you can post to the community by mentioning it