Even online shops really should have care info, unfortunately a lot of them don't. Had the boyfriend come home out of the blue with a couple of clown loaches for a similar sized tank. They're very active fish and will quickly outgrow it, I had an excuse to get a larger tank but I'll likely have to rehome them soon. I also had a freshwater clam for about a year and wouldn't suggest keeping them. They're very difficult to feed since they're filter feeders and you likely won't be able to provide enough food for him. You could try getting some special liquid food for filter feeders and squirting it at him with a pipette, but unfortunately, they often slowly die of starvation.
Here's what I'd do to sort out your stocking:
Rehome both the clown loaches and the dolphin, you should be able to find a LFS near you that will take them, but you can't expect to get much if any of your money back no matter what you do. I've had to rehome several $20+ fish before and never got any reimbursement though I do know that they all found happy homes.
If you want to keep your betta (fighter fish), get yourself a container that holds at least one gallon of water to house him, the bigger the better. You'll want to do frequent water changes on an unfiltered bowl and could technically keep him in there though you may want to consider getting a 2.5 or 5 gallon filtered tank to house him in eventually.
With your four guppies, you have a few options in a small tank like that.
This is a good thread to give you some idea of species to look for. I'd go for one type of bottom feeder and possibly a honey gourami or a trio of sparkling gouramis (one male, two females). If you went with all guppies or similar sized fish, you could also keep some algae shrimp like the cherrys or amanos.