Radish… I never thought of growing it in modules, sounds promising, certainly it should do well as a transplanted clump of three or four together. Having said that, radish is such a fast growing plant that I usually sow it direct, and soil is bare at this time of year, ready for sowing.
Onions do work well in modules, four to six plants in each. Bulb fennel work, so do swede, both sown two seeds per cell and thinned to one. It is fun growing this way because you can keep a close eye on seedlings and bring them through their most vulnerable stage of life. Good luck and let us know how it goes.