Gouramis and red sharks are both aggressive fish with some specific care needs. Too bad the petstore sees the fish as products to sell instead of living things. They'd never sell you two puppies that might kill each other. What a shame.
You've learned a tough lesson about pet stores. Advice first, petstore second, because you won't get it once you get there.
I made the same mistake with gouramis - got 2 males for a 10 gallon tank. One of them shouldn't be in a 10 gallon, let alone 2. They are nasty things when they want to be. I'd take both fish back and get some more peaceful community fish - rasboras, cories, platies - all can live with a betta if that's what you want. Some varieties of tetras, some mollies. Do some research first, you have a lot of options. Water conditions and tank set-up can make a HUGE difference in what fish wil thrive in your tank, so when asking for advice on which fish to get always make sure you post tank size, water parameters (if you don't know, then you shouldn't be getting fish. Pet stores will test your water for free, or invest in a test kit), what fish you currently have, and what kind of decor/plants/substrate you have.
Good luck! It's a great (and addicting!) hobby, but it is a lot more involved than most people think it is.