Oh in that case it's probably fine if it's just a small amount mixed in. I thought you meant giving just plain barley or clover hay. I would feed that mix as a secondary hay though, so mostly timothy hay but with some of that hay given a few times a week.
The reason to limit it is, too much barley (grain) can cause gas, which can lead to bloat, and clover is high in protein and calcium, and too much of those can lead to bladder and kidney stones.