Main question: Well, um...With the current version, using custom textures hasn't been added...As far as US Legacy, I'm pretty sure it's possible but I have no idea how.
PS question: Probably a bug, and you can't make somethings radius 9 times its diameter, thats just not how it works XD But otherwise I'm not sure what you mean, but again, its probably a bug. As far as the black holes go, the limit depends on the objects mass and the current gravity strength so there isn't really a hard limit, just the object's Schwarzschild radius: The point where an object's gravitational pull grows beyond the strength where light can escape, and in the case of US2, an object becomes a black hole if the Schwarzschild radius reaches the object's surface or if the surface shrinks to become smaller than it.