Oh my god those people are horrible!
Yes, they lied to you a lot. They have no responsibility in the fish market and it sounds like they just look forward to you restocking your fish constantly from their poor advice.
I'm no expert, but I'll try to help how I can...
First, a good filter does help a lot. You do need to change the filter media eventually though, I go every two weeks. How fast yours builds up, and your stocking levels may change that though. Also keep in mind I have 32 fish in my tank with four filter media between two dual units, rotating each week what one switches so I don't upset balances in my OCD self... Anyway! Your expirience may differ in that case, when you do switch the filter tear some of your old media off the card and put it in front of your new card (before the card, not after when it flows through to the tank.) This will help spread beneficial bacteria to your new filter media quickly and keep you from breaking your cycle easily. Some filters have things like biowheels or other bio media that make a "home" for bacteria to help in that regard too.
You do need to do water changes, but its really not that bad. Once your tank is established id aim for a 10% water change every week (or every two weeks depending on your flexibility). Petco sells a really easy syphon that hooks up to a bucket very easily. Just shake a little and it starts pulling water out, run it across the gravel (about half the tanks floor each time, try to avoid overcleaning by doing the entire thing) then the next time do the other half. This removes fish waste and extra rotting food. Replacing water is key in maintaining a "healthy" water system for your fish, just be sure to get a conditioner for tap water first.
Bettas and guppies can survive together BUT, it all depends on how territorial your betta is, and how fancy the guppy is. Avoid bright colors or big fins on guppies for roomies. I gave my Betta two snail room mates in his tank.
Don't worry too much about being overwhelmed, community groups are all over to help
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