I would plan on doing a 50% water change every day to keep the ammonia level within the boundaries of acceptability to be on the safe side. I did this with my betta and he did fine. But the better choice would have been to have the tank cycled first or to have some filter floss or media from a cycled tank to put in your new filter to have the tanks new filter already cycled from an established tank.

Your new betta will not probably like a full finned guppy so I hope that this guppy is not a fancy tailed one. Otherwise they can confuse them with another male betta. One gallon is also not a lot of room for a betta to move about in no matter what the people at the stores tell you. You may want to consider doing an upgrade to a larger tank in the near future if you can. They do really much better in a 5 gallon or larger. You will be surprised in the increase in the activity level of your guy.

Rose

For future posts there is a wonderful source for show quality bettas in Miami...they sell through eBay Miami Aquariums.

They have a website at http://www.miamiaquariums.net and can be contacted directly for both freshwater and saltwater fish and bettas. I have gotten lovely and very show worthy fish from them and they have done the quarantine and worst of the process and the cost is not any higher or perhaps not as high as the import and all from the Thailand fish.