Yeah, most of the time the pet store workers don't know anything. They either have fish and have been misinformed as well. Or they get their information from the store. I now work at a Petsmart, and 3/4 of the people who work in the fish department (including me) actually know about fish. I was quite surprised. But yeah, always take their information with a grain of salt. Do your own research. And make your own conclusions. The tags about each fish at the store have tank requirements and such. Very few are alright. We have pacus (which get 2 feet long!) that say they only need 20 gallons! I don't go by those requirements and neither does anyone else at my store. And wow Pitluvs! That's crazy. My manager wants to be an ichthyologist, and knows a lot. I have made so many recommendations for people to NOT buy stuff (especially fish). And my manager agrees. He cares about the quality of life of the animal, not the sale which is great and I don't have to worry about giving correct information.
But about tankmates, I have a betta in my 29 community with platys and peppered and albino cories. He's completely fine. He actually likes hanging around the cories when they eat. I had a different betta in there before, but he got too aggressive with them. It depends on the individual fish and the species you want to house them with. Bottom dwellers are usually ok (assuming they won't outgrow the tank). Smaller rasboras and platys can work. Any semi aggressive is a no. Gouramis are related to bettas and will fight. Certain tetras and mollies tend to be nippers. I have heard of them being with guppies, but I would be extremely careful about that because of their long colorful fins might be mistaken for a betta. And of course snails and shrimp (at the risk of possibly becoming a snack) are fine.
Tanks- 55 gal(in progress)-Common pleco, Angels, lemon and rummynose tetra, blue rams; 29 gal planted- rasboras, corys, otos, clown plec, shrimp; 10 gal- juvenile goldfish; 6 gal planted- betta
Other Pets-Maggie(Tricolor Paint Mare), Lily(Sun Conure), Zoe and Melina(Budgies), Leonardo- Bearded Dragon, Pilot and Phoenix(Golden Retrievers), Beckham(german shepard/akita), Myrtle and Richard(hermit crabs)
A Better Betta Fish Care Video!