Regarding a definition, I'd try to look for a succinct one, as longer text in the sidebar tends to look busy and overwhelming. Maybe alongside it a few examples of libre projects/media, say stuff like Project Gutenberg and LibriVox to creative software like Krita/Godot/Blender/etc. Examples would give a much clearer idea of what's considered libre culture to discuss than a definition alone.
Also I think the idea of clarifying what sort of posts are allowed/desired is good, and speaking of memes, some restraint may be wise to avoid it becoming simply a meme community. Maybe a semi-regular memes thread or something? If said threads were active enough then you would have a better sense of whether or not setting up another community dedicated to them is worth it.