The barn has a very American feeling in the design. Most European barns would be more open in the middle ... the four centre posts limit storage possibilities ... so would steer this more towards stables. This may not be a bad thing, as it would allow you to accessorise the barn easily with stalls and horse gear.
If the base does not sit properly on the poured hydrocol, maybe adding a frame at the bottom and adding putty to the frame and sculpting the stone work wold do the trick?
I really like to wooden floor ... the sanding of the joints gives a nice rustic effect, but maybe too exaggerated for this scale? But adding some straw on the floor, and what will be seen through the doors, this will probably look perfect.
Above sounds a bit harsh but I mean it all constructively. What you have done so far is excellent. I must look into getting some of that weather all product. The effect it leaves on the floor board is tops!
Looking forward to more progress images.