Neon tetras need to be in groups of 6. 6 neons and a betta fully stocks a 10 gallon.
Mollies (in my opinion) get too large for a ten gallon, and three would fully stock one. Although they can live in fresh water, they do much better in brackish, which would not be nice for the other fish.
The sucker fish is a serious problem. Without identification, it could be anything from a bristlenose pleco, which get 6 inches long, have a large bioload and need a 20 gallon, to a Chinese Algae Eater, which also get large and grow very aggressive as they age. Can you post a photo?
I would strongly recommend taking the mollies and the catfish back and buying three more neons. However, I wouldn't buy the new neons until the tank is cycled. Cycling is the process of growing good bacteria in your filter. Without these, the filter is only partially effective. The cycling process takes about 6 weeks, done fishlessly, more with fish-in.
By adding all the fish at once you may cause an ammonia spike, which could harm your fish, so keep a very close eye on ammonia levels with a liquid testing kit and do as many changes as necessary.
I'm sorry if this comes across as harsh. It isn't meant to be. :) We all make mistakes when we start out, particularly when taking advice from petshops. In future, though, always research or ask here before committing to buy, as there are many factors, some of which I haven't mentioned, that need to be taken into account when buying fish. :)