it dont think a tank with shrimps would look good without plants.
a planted tank is certainly a must with any invertabrate tank.
eschaton idea sounds a good one, not sure about the Cajun Dwarf Crayfish, if its allowed in the uk or not, but researching it, it looks a very intresting invertabrate. i would be careful about the apple snails too, the Brigs i think would be ok, but the Spixi may eat plants.