I would personally not keep a betta splendens below about 22C. I know full well that a betta can survive and thrive at that temperature. Since many of our homes are that warm in the summer months, you will have plenty of time to locate a nice small heater before fall. The minimum volume for a betta splendens is debated at length both on our forum and others. The factor that is seldom part of those debates, they are hardly civil enough to be called discussions, is that maintenance practices are the real thing that determines minimum tank size. A simple gallon container is plenty if water changes are done often enough and are done properly. "Jarring" bettas is a practice followed by any commercial Betta splendens breeder. Using huge volumes to care for the males cannot be supported at a practical level. Since the livelihood of the breeders depends on them delivering top quality fish to the retail market, they never take chances on using too little water but they also cannot afford to waste water. It is a fine balance that they strike. Most hobby situations, outside of breeding programs, again will use something between 3 gallons and 5 gallons as the minimum that is found acceptable. We do not do the major frequent water changes that betta breeders must do. The debate will undoubtedly continue but there is no such thing as a gallon container that cannot be used for a Betta splendens. The question, as always, is best measured against the maintenance regime being used.
On the other side of the argument is often people who would strongly disagree with my own approach to keeping bettas. I place them into a community situation of considerable size. Right now my oldest male betta is in a planted 45 gallon endler tank along with a few bristlenose plecs and some cories.