Room temp is the preferred HIGH range for most species. Some will tolerate warmer, but most simply tolerate it and adding a heater will only stress most species. Really just think of setting up a basic, low-tech system. If it is a luxury item, your natives dont need it. Golden shiners get to a good size and can make an impressive dither school to add color and movement. *Good suggestion warmouth*
IME, Many panfish will take territories. Dominant fish can be quite aggressive to the subordinate fish. Simply breaking up lines of sight and "sectioning" off parts of the tank with careful landscaping can cut down or eliminate aggression. Once a dominate fish is established, his territory is the important one. The others will play nice in a school, usually to avoid the aggressive dominate. Fascinating social dynamics and behavior