Room temperature.
Betta fish do not enjoy currents that come from filters. You could put in a filter but I would recommend not to. Also, Heaters are unnecessary as long as you keep the water 70-80 degrees.
No
If you betta is in a small tank, placing it in a location that has a warm ambient room temperature should be enough to keep it to betta preferred warmth. If you do not, then you should look at getting a small heater for your tank, but you may need to get a larger tank to allow for the use of a heater as there are no heater models made for anything smaller than a 2.5 to 5 gallon tank.
A betta fish can live in water set at room temperature, but anything below 68 will start to become fatal.
72 - 80 degrees Fahrenheit
With anything new to it's environment, a betta fish will be cautious of anything new to it's tank (this is because of their territorial traits). But depending on the betta, it will take a day or so for it to get used to the props/scenery. But don't be surprised if the betta stays clear of a certain area, he/she may not know if it is safe or not.
Pretty much anything including other fish
78-80F. They are high-end tropical fish.
Because the female will usually try to eat the eggs. He will fight to protect them from anything, even his mate.
It is unsafe to heat anything smaller than two gallons. As long as your room temperature is reasonable (68-76ish for a betta) you should have no problems. If not get at least a two gallon tank and heater, one with an internal thermometer unless you want to monitor it 24 hours a day.
no way, it doesn't know how to hunt or anything!!:)
Personally, I wouldn't keep anything in a 1.5 gallon tank. Not even a single Betta. I consider 5 gallons to be a minimum for a single betta.