How does one typically blend their meads & wines? Do I need to worry about layering because of different S.G. levels or ABV levels?
Never had a problem with layering when I blended, but both "brews" were within 0.010 spec grav of each other. Blending each at a relatively high temperature (100o F) should help reduce any layering. After the mixture is made (and homogeneous), there shouldn't be any separation. I've made quite a few mixtures of two liquids at significantly different spec grav in my career and warming seemed to reduce any separation on cooling.
Essentially, you are mixing two solutions of alcohol and water and ethanol has one of the largest suitabilities in water. Vigorous swirling of the warmed "brews" should do the trick.
some additional info: Elevating temperature increases the kinetic energy of the molecules (in this case) and also increases the distance between the molecules. This allows other molecules, especially large bulky flavenoids, to more easily fit in the spaces between the molecules (solubility). Specific gravity is a measure of relative density (can be thought of as molecules per unit volume though the formal def is weight per volume) and when two liquids have very different densities, they will not mix easily unless mechanically stirred.
There will be zero layering. You are combining two water based solutions, they mix completely as mixed. Gravity is meaningless in this example as is temperature. Ethanol is completely dissolved in the water component of mead and cider. Remember too that mead, cider, wine and beer are mostly water!
Just blend away.