brrrrrrr... don't attempt to fit textures into existing UVs, won't look good
Better to make something yourself, even though it might be simple! By that I mean create some simple, easily uvmapped geometry in a modeling program and fit textures into that.
One should also be careful when taking textures from cgtextures and just generating a normal+specular+gloss for them. Will turn out noisy and 'wrong'.
Ultimately it depends on what you can pass off as 'okay' depending on the amount of work you are willing to put in.
I could make a 'basic tutorial' for this if people are interested in this sort of thing.