You'll need to perform a water change every day or every other day, depending on how high the ammonia and nitrite levels are. Up to a 50% water change won't harm anything. You'll know when your tank is cycled when the ammonia is at 0, nitrite at 0, and your nitrate levels have significantly increased. If you don't already have one, I suggest you buy a master liquid test kit to use since the test strip kits aren't accurate.
Monitoring the pH level has nothing to do with cycling the tank. Neither does adjusting the pH or nitrate level. You need to test for ammonia and nitrite quite a lot to monitor the water parameters for your fish. Running a filter/heater for a couple months is going to do nothing for a cycle; if there's no ammonia in the tank then the bacteria isn't going to grow. Since you already have your fish, you might as well keep them as long as you're willing to keep up with the water changes.
Also- please do some research before believing the posters on here. Bettas can most certainly live with other fish. It isn't a guarantee that the guppies will survive due to their fins, but I've seen them live with bettas before and be fine.
Sure water is water, but when there is AMMONIA and NITRITE in the water, that's not safe water. The nitrogen cycle needs to take place with the help of a beneficial bacterial colony (not plants) in order to convert the toxic ammonia and nitrite in the tank into nitrate. Salt isn't necessary in this situation.