Can anyone help out and tell me, where I can find the associated models? Or if they would probably work on one of the default civ5 models?
You're apparently looking in:
Advanced Naval Units (v 9)\Surface\Light Cruiser\
which contains a couple of units. Looking at the texture sheets alone is misleading, as it looks like the ironclad is using the files you're asking about just for the cannon pieces (hence the filename). The texture itself comes from the Civ4 Frigate, which AFAIK hasn't been converted to CiV.
FWIW, note that Advanced Naval Units has a USS Constitution based on the War Galleon in:
Advanced Naval Units (v 9)\Renissaince Era\Surface Ships\Constitution\
As above, don't be confused, as a model doesn't necessarily use all of the textures in a given image.
As I don´t have any idea how to extract the vanilla art files from civ5, I can´t try it by myself.
As far as tools:
The SDK includes a program called Nexus that, among other things, contains scripts to extract the .fpk files. You can also use the third-party tool
Dragon UnPACKer.
Nexus can also be used for viewing gr2s with their textures outside of the game (you can easily use RAD Granny Viewer to examine units, but without their textures since Firaxis uses non-standard Granny)--except Firaxis broke the Nexus 3D Viewer at some point in development and never fixed it, so you'll need to use
the old version of the SDK if you want a working 3D Viewer.
Going further down the rabbit-hole, if you want to reskin a unit (i.e., make a copy of a unit with a tweaked texture file), note that the texture references in the .fxsxml file don't really do anything. You'll need to edit your copy of the .gr2 to point it to your new texture. You can use the community-made tool Nexus Buddy (either
the simple old version which is sufficient to change texture references, or
the full-featured rewrite, NexusBuddy 2, which can sometimes be more difficult to get working, but is a lifesaver if you're doing unit conversions).
It's not relevant to units, but note that if you're interested in looking at the UI textures that include an accompanying "*-index.dds" files, you'll need to use
this tool to convert them to png.