Seriously.
XNova is a GNU licensed product, that mean you can use it for whatever you want, you just need to give credits to the creators.
Anything about renaming things is dumb.
Gameforge should be the first one to have copyright problems, many (not to say almost every single one) of the images of the main skin were ships from other games (for example, the Bomber was an advanced fighter from the game "Wing Commander Prophecy").
What you need is to use XNova material, if you really wanna be sure you won't have problems, use differnt images made by yourself or some guys on sites like deviant art who will give you the permission to use them, but renaming things is useless. Heavy Fighter? I don't think it's a Gameforge copyrighted name... Battleship, Destroyer, Cruiser, all those names are free commonly used words and can be used without any problem by anyone.