Someone else will have to tell you about cycling. There are a couple of threads about cycling in the betta section of the forum. Yes, you need to use water conditioner. I use Aquasafe by Tetra. Other people use one called Prime but I don't know who makes it.
You'll want a 50 watt heater for a 10g tank. Note that keeping betta sororities is something of a risky business. You must have a very heavily decorated (i.e. planted) tank to create enough hiding spaces, and even if you do everything right there's still the chance that your fish simply won't get along. Whenever attempting this, always be prepared to move fish to separate tanks if needed.