Are custom downloaded tanks usable with online games with the new version?

No, mod tank.xml file needs to have your custom tank or its not usable.

Armor

The issue is now we have tank types that allow tanks to have all sort of attributes that can be associated with each tank. e.g. each tank can have different health, weapons etc... Allowing people to define their own would break the ballance of the mod (and could be considered cheating). Of course we could just allow people to skin (change the look) of the existing tanks but thats harder for people to configure.