Depending on where you are getting your fish from commons actually may be harder to keep than exotics. Lfs brings commons in for next to nothing, they know then can move them fast because they sell it cheap, so they don't really put too much effort in keeping them. Exotics, in another hand, costs lfs a bit more to bring in, so it's in their best interest to try to keep them healthy (ah ha! finally I disagree with David R!). Too bad I'd have to agree with David R Again on the no beginning fish concept. If you must say there are types of beginning fish, I'd have to say it would be based on the pricing of the fish, not difficulty. As long as you've done enough research, and are willing to provide the correct environment for the fish, can buy healthy fish from good source, most fish are fairly easy to keep.
Don't buy a common just because you think they are hardier or easier if that's not the fish you want to keep. You'll just end up having to deal with trying to sell/give it away. Buy something you like, buying from the market place is usually a safer bet then from lfs.