Grasses protect soil during the winter and also produce great forage or plowdown organic matter. Legumes do not grow well during the winter, but they grow abundantly in late spring and produce high protein forage and lots of nitrogen as plowdown for the following crop. Crimson clover comes first to mind in choosing a legume to grow with a grass. Crimson's height matches well with barley, wheat, and oats, but it may be shaded by rye, resulting in less growth than desired. Hairy vetch has been sown with grass cover crops for many years, using the grass and vetch combination as a hay or plowdown.
Many growers plow down winter cover crops in late winter or very early spring.
Try to resist this temptation until cover crops have gained sufficient biomass.