If you want to avoid a shading problem, plant your oaks on the NORTH side of your plot. The oak roots will take a lot of moisture out of the soil, especially as they mature - can't stop that.
If you want another type of clover to add to your mix - try some Alsike clover. It does well on poorer soil, and will also fix some nitrogen into your soil, as all clovers do - being legumes. We've used Alsike clover at our camp on newly turned soil that was fairly acidic, and it grew pretty well. Alsike may be a good addition to what the other guys have recommended, if you want some variety in your mix.
We also plant buckwheat. It needs warm soil to germinate, as someone pointed out earlier. We usually plant it in mid-to-late May and it grows well, attracts lots of deer. Good soil builder and weed suppressant crop - IF planted heavily enough. BW attracts turkeys like candy when it goes to seed in late August / September.
As someone else said - rye GRAIN - not rye grass - will grow just about anywhere. No muss / no fuss. Great winter food and first thing to green-up in the spring. We use it at north-central Pa. mountains camp. We plant at a rate of 150 lbs. / acre for a pure stand, and 50 lbs. / acre for a mix of other seeds.
