This is where you would use the equal diameter construction circles. Center the vertex of the rectangle and tangent to the outside line. This way, you didn't dimension it, but whatever the radius, they are equal.
Otherwise yes, it would be over constrained because the equal dimensions are after all, still dimensions.
Can you make reference dimensions equal?
The equal diameters are really no different than equal dimensions. The issue in this case is that he can't create the two dimensions (in order to later set them both equal) because doing so will overconstrain the sketch. By temporarily freeing up some other part of the sketch (like the corner constraint), the dimensions can be created and set equal, then the extra dimension deleted and the corner reconstrained. And no, you can't set reference dimensions equal to each other.
Not exactly. The difference is that the construction circles are -not- dimensioned, therefore their diameters "float" equally. You can do the same with construction lines but circles are easier to manage.