I personally think texture is easy to over exaggerate in the smaller scales. You can't see it with the naked eye.
That said, I do think the structure above looks unnatural. First of all, I think the gray is too dark. It doesn't look like a natural stone, and it's far too uniform. A sandstone or buff color would look much better. Then a wash of black to take the uniformity away. You can even add two different washes, a slightly lighter and slightly darker version of the base color, just to give the stone some variety. That and some streaking of dirt and rust would make it look better.
If you must add texture, one way is to paint a base coat, then sprinkle on a very fine powder, like baking soda or fine sand (talc is probably too fine, but might work, depending on your scale), while the paint is still wet. Let it dry, then paint another coat over the first. Some might recommend textured spray paints, but I think those would be too chunky.
Connecticut Valley Railroad -- A Branch of the New York, New Haven, And Hartford
"We don't stop playing because we grow old. We grow old because we stop playing." --George Bernard Shaw