I occasionally come across content in my feed that I think would be of interest to one of the small communities that I participate in. Think photo or article, not something personal. Is it appropriate for me to crosspost the content? Without asking the OP? What about if I ask but the OP never responds?
Linking between instances is kinda weird right now, apparently. There's no good way to link to a specific post in a way that's accessible for users across instances.
Post whatever you want wherever you want, but credit the source. This is important not just for courtesy, but for archival purposes. It leaves a trail that makes it easier for others to find the original if they want to see other work by the creator or information about the subject.
If it's just a meme, it's probably not going to matter much, but for original artwork or articles, the author should always be credited when possible, and links make that easier for everyone.
I would take it as a complement if it is something I posted and it is a person just adding to a relevant community. Just don't spam post it to several communities at once or within a short time frame.