Either the Betta or a single ADF will be fine in a 5G. Not sure of location; but lack of heater should not be a problem now, it's been in the 90s the whole week at mine. If no filter just replace 25% of tank water daily with dechlorinated same temperature water. Once get filter, it will take around 30 days to cycle, so keep doing water changes every couple days to keep ammonia and nitrites low. Good luck !