Yes, do not go get another guppy unless it is the same gender they reproduce really fast. But fish are better with another fish.
2 or 3
No, you cannot put these two fish together. In general you should keep Beta's alone by themselves, not even with other Beta's. They are mostly kept in bowls without a light, a heater or an oxygen line. The Beta will kill your guppy.
no
You can keep endlers guppies in a 1.5 gallon, about 2, if you maintain it (take some water out, take uneaten food out, ect.). you should keep the tank planted well because they enjoy it. You can buy endlers at petsmart. :)
As long as you have your tank heavily planted, the frogs should be fine.
yes a light is enough to keep a guppy warm just make sure you focus it over the whole tank
If you are worried about a guppy aborting you must be doing something wrong. Guppies are so very easy to breed. Just leave her alone feed her regularly and keep her water in good conditions (regular water changes and temp around 75F) and she will provide you with healthy young fry.
Tail
He wants to mate with the female. That is a guppy's normal mating behavior.
They can survive, but they will not thrive. You should be looking at something 5 gallons+ if you want your guppy to live a long and happy life. Keep in mind that they are tropical fish, and room temperature will not usually be warm enough for them.
It depend on what species you plan to keep and how long you plan to keep it.
They do best in temperatures between 75 and 85 degrees.