You might get by on a wall, but there's a 2" minimum size specified for tile when used over Ditra.
There are two ways to handle a shower wall...you either use a moisture barrier behind it, or waterproof the front entirely from the waterproof pan up at least to the height of the showerhead.
What are your overall plans on the use of RedGard? If you're planning on using that to waterproof the walls, then you'd not use a moisture barrier behind, and to ensure the edges of the Ditra were not open for moisture to get behind it because of the waffles and the fleece, you'd need to use Kerdiband on the edges. RedGard would not seal those edges and prevent moisture from getting behind it.
FWIW, once you thinset the Ditra in place, if your tile really are 1/4" and 3/8", the mosaic will likely sit higher than the field tile. What some do is get an 1/8" trowel, trowel thinset in that area where the mosaic will be installed...let it cure, then add thinset on top of it to set the tile. IF the area where the mosaic is going isn't too wide, you can tile above and below, then using those tile and a grout float, or something big enough to bridge those two surfaces, set the mosaic deep enough so the top surfaces are flush. The first coat and screed with the trowel will act as your height gauge. NOte, if you don't hold the trowel perpendicular to the wall, the peaks will be less than the 1/8".
Check out
www.johnbridge.com for help with tiling.