You can participate for all communities if you want, but don't use the tags on the same threads because the topics are not the same on each of them, it would be spam. Different threads for different communities.
You don't need the tag on the replies because if the main threads has the tag, the comments on that thread would be counted. You can, but it won't have any benefit.
Same with the comments on posts.
Okay, well understood. Thanks.