If it's published by Image, or an imprint, it goes on this site and it's permanent. We don't get rid of content when it moves. It was on Image, it gets documented by Image Database.
Volumes are respective to Image published materials only. I've spoken with other publisher admins and we agree there's enough information in the volume description that people can figure it out.
Non-canon source can be referenced if critical for explaining canon material but should be called out.
If issues are bought, if Image is going to print it, and no active wikia exists for the material, this is the grey area. We should point to the wikia authoritative source if it exists. So characters from DC, Marvel etc should point to those wikias and not recreate pages for them. Alternate option is wikipedia pages. If image is going to be the only print publisher, we can cover it here.
I owe John and the community a policy on this though. I think it's cut and clear 99% of the time. John recently expressed concern on another wiki about this and we owe it to start working on this. I think there's fine tuning to happen, but it hasn't been a priority or issue to bring up for us since the 90s and early 2000s which tends to be John's area of expertise.
In the mid 1990s, Liefeld had a publishing imprint called Maximum Press, which was the first company to publish Avengelyne. Maximum briefly published Supreme and some Youngblood related titles, before Liefeld started another new company, Awesome Entertainment. And shortly after Awesome folded, Liefeld had yet another company, Arcade Comics. And now he's publishing his characters through Image again, basically because he's friends with Kirkman and Eric Stephenson.
ABC (America's Best Comics, owned by WildStorm and DC Comics) never published Liefeld's characters, rather, Alan Moore took the ideas he developed for Glory, and transfered them to a seperate series called Promethea, which was an ABC title.
So, do we cover the comics published by Maximum, Awesome, and Arcade? I say yes, because the Liefeld characters are integral to the Image superhero "universe".
Likewise, what about the Mike Allred Madman titles not published by Image, or the Powers issues published by Marvel/Icon, or non-Image appearances of the Maxx? Or Anti-Hero, which wasn't published by Image, but is part of the Faerberverse?