@KoalafiedPonki hello and welcome to the fediverse (and to kbin).
Lemmy, Mastodon, and Kbin are three different "apps" or software that access the "fediverse". Lemmy is like reddit, Mastodon is like twitter, and kbin is like a mix/combo of both. All three can interact with each other.
Each of these apps have "instances" which are a bit like an email website (gmail vs yahoo). You sign up to one website, and then can interact with the other websites. For instance, I'm on kbin.social and can speak to someone who is on lemmy.ml or lemmy.world. Similarly I can talk to mastodon users who are on mastodon.social.
The sites are sharing content/posts with each other so we can interact. but some sites/instances may block another for various reasons, at which point they wouldn't see each other's content.
You only need one account for most purposes.
As for "is this post allowed", let me clarify what "this" is. You've posted a "microblog" here on kbin under the "redditmigration" magazine. For mastodon users, they simply see this as a "tweet" you made (as if you were on twitter). For kbin, we see it under the "microblog" section of the "redditmigration" magazine. The ettiquette is... yes microblogs are more personal so you're free to use them as you wish. threads work like how they do on reddit.