Ellie, I think white clover is one of the best forage crops, has very little coarseness to it, and is about the right height for chickens. That "pasture mix of Rye, fescue, orchard grass and white clover" looks like a good choice to me.
I once lived far out in the country and planted a very similar "pasture mix" for my lawn. The chickens loved it and I could mow it quite close. It looked wonderful even tho' not quite the same as a suburban lawn.
BTW - caveats are due for the Cornell toxic plant list. For example, they list alfalfa as toxic to "cattle, chickens, humans, sheep." So, how could that be since alfalfa is grown for hay as a Winter feed for cattle and sheep, found in formulas of chicken feed, and enjoyed as a tea by some of us humans? Cornell should probably explain that some of these common forage crops are "toxic under certain situations." Most everything could be toxic under certain growing conditions or when consumed in great quantity. Chickens are awfully good at self-regulating their diet if given a broad enough choice.
Steve