Pet store employees have only the basic knowledge of keeping fish. For the majority, it is not their hobby, nor their passion. It is just a job, and their job is to sell you fish, tanks, and supplies. They're going to tell you what you want to hear.
Your best bet is to disregard what they say and do your own research, or at least take what they say with a grain of salt. I've had more than one experience with horrible pet shop advice.
Fish keeping can be quite tricky in the beginning, same with when you first keep any new pet. There's always a balancing act, learning new things, and making mistakes. (Such as leaving your brand new pair of shoes out after you get a puppy. One who likes to chew.)
It's best if we start from the top and work out what you need, what you don't, and what's happening.
For any tank, no matter how small or large, there is a 'cycle', that goes on. This is the process of fish waste (ammonia, which is highly toxic to fish), being converted into nitrite (which is also highly toxic), and then finally into nitrate (which is non-toxic in low concentrations) by 'beneficial bacteria'. Unfortunately, many pet shop employees consider low numbers of ammonia/nitrite to be non-toxic, but this is in fact the opposite of the truth. Exposure to ammonia and nitrite of any level has disastrous to fish, and will eventually kill them. There is no 'safe' level of ammonia/nitrite besides zero.
Your best bet is to buy your own test kit, where you can test the water yourself, at any time, and have the numbers right in front of you. The 'API Liquid Master Test Kit' is invaluable for this, and costs about $20 on Amazon. It will last you for years, and is just as important as the water itself in my opinion.
Unfortunately you are in what we call a 'fish in cycle'. The only way to keep your fish alive at this point is to be changing the water every single day, half the amount, and using a good water conditioner, such as Prime by Seachem. This will detoxify ammonia and nitrite for up to 48 hours, keeping it from building in the tank until you do the next days water change.
You may also look into returning the fish, and doing more research and deciding if this is what you really want to do. Fish do require care, and it can be tricky getting it down at first. But after awhile it becomes second nature. I have three tanks, a 38 gallon (my first), and 20 gallon, and a ten gallon. Maintenance on them takes less than an hour a week, for all 3.
Another great thing to add would be live plants which basically 'eat' ammonia/nitrite, but we would need to know the light on the tank before we delve into that.
What the store is not thinking about is how active guppies are. They really like their swimming space, and can be quite aggressive. When they are confined to a small space, this will stress them, as they can't really 'get away' from each other, nor do they have enough room to exercise. This stress with compromise their immune systems and leave them even more susceptible to the ammonia and nitrite building up in your tank.
After you get through this rough patch, it's good practice to change half the water in your tank once a week, to keep organics and nitrate from building up. Not the 25% monthly they told you.
Just to comment on the thing about the betta- that fish is not thriving. He is merely surviving in those conditions. A betta kept in a properly heated, filtered, adequately sized tank will be active, robust, and personable.
EDIT: All fish require basic care to survive. Basic care is an adequately sized, filtered, heated, cycled tank with weekly water changes done. With this provided, there are many types of fish that can be kept. But first, before suggesting fish, we need to get through this cycle and deciding if you will get a larger tank or not.
