Work in progress, a mosaic of a hare from a later Roman mosaic. I'm posting this up to show you a very good example of a piece that has been set out in such as way as to make it as straight forward as possible for the mosaicist. Although the photo appears a little small here, notice how in the original mosaic the distances are set in full tesserae. The ears and legs show this and the face is effective yet very straight forward.
One of the easiest ways I find to check if you design for your mosaic is getting too complex is to ask yourself, 'If I had to do another 10 examples following this template would it be easy for me to follow?' Don't make life difficult, wherever possible use full tesserae widths, narrowing only where you need to. Don't have too many areas where you get sections with a width of 1 1/2 lines, do this and you could end up having to cut every tesserae in the line. Also show the detail you want in faces but keep them as simple as possible. You show more detail by using smaller tesserae, across the whole feature, not by cutting lots of different shapes.