Looks good tiling wise, I'm not too much help with the other bit
As far as big highlights being bad, they're not, it's more that things that stand out are bad when making a mass used tile, if you decided to put moss on one of the stones you would be able to pick that out when tiled and you would see the grid, you would normally have a few extra tiles that look different so you can mix them in with the normal ones.
