These are my advices:
Black Knights invade in Kiln, ToG, and Dukes Archives.
Silver Knights invade in Anor Londo and Painted World.
You can use any weapon/magic you like but you MUST use any of both sets.
If you need to have low weight or you have little endurance (which is bad, in my opinion), you must at least use the helmet or the armor and the boots (preferably the helmet).
Silver Knights would be well for low level people, and Black Knights for high level people/people who are in kiln already.
If we have to kill the 4 lords that doesn't matter, it's a detail, lets not stop this project for such a tiny detail; after all, we do it for a good cause lol.
You should have a way to identify between us (like a gesture or a gesture and a prism stone), so if we invade one another, we don't kill each other, and the invader uses Black Crystal.
I think that any covenant could join, as long as they can invade (even if they have to use cracked red eye orb), or in the case of gravelords infect people in their respective areas.
^I see no flaws there.