I would read up on the different fish you have, search their names on google and read sites explaining how big they get and how to care for them. I especially recommend this for the angelfish, mono, and clown loaches. Most likely you will need to take those back to the store if you don't want them to die, or get a large enough tank for the loaches and angelfish (a 55 gallon or larger), and a large enough brackish water tank for the mono. The clownfish can get more than a foot long at adult size, and the angel will need a deep tank with slower moving water so it can swim with its long fins. If you trade them in, ask here for advice on other fish that would be better tank mates for your danios and guppies.
I'm sorry your local fish store people told you those would be okay in your tank. They often give bad advice either because they are trying to sell more fish, or they simply don't know any better. Check in books or on websites like these instead of trusting the local fish store (lfs). It will save you the trouble and disappointment of losing your fish.