In all honesty, none. WCMM are small, but they're very strong and fast swimmers and really need a tank that is at least two feet long, preferably three for them to be happy.
I agree. Get a little 25 or 50W heater, and there are loads of options of small tropical fish that would be happy in an Edge; Endler's, dwarf emerald rasboras, celestial pearl danios, ember tetras and mosquito rasboras would all do well.