You'll need to do weekly water changes IMHO - not every 2-4 weeks.
What you basicaly described - fun, playful, interesting, etc - is a corydoras catfish. They are common fish but very cute and you'll love them once you've kept them for a while. They are also considerably hardy and don't get big. They like to be in groups of at least 4 of the same species and you'll notice they are very social and interact with each other like very few other fish do.
Concerning the fish you named, I understand you are looking for unusual but none of those would work in your tank - the shark catfish gets too big IMO, the elephant nose is not a beginners fish as it's very fragile and requires a species tank to thrive and, though the dwarf puffers WOULD work, they are best kept in a species tank and are not realy suited to community life.
Because you are new to keeping fish, I'm going to suggest a very peaceful community -
As I've already mentioned, 5 cory cats would be very nice for your tank.
An unusual and interesting top-dwelling schooling fish is the silver hatchet. 6 of these unusual creatures would look very nice IMO.
To mature the tank (your first inhabitants) you need something somewhat more hardy - I suggest you get a trio of platies. These are not particularly unusual but they come in a rainbow of colors and if you get a male and 2 females they will breed for you. Platies would add that extra bit of color and before long you'll have more than 3.
Lastly, a schooling fish for the middle layers like glowlight tetras or black phantom tetras (wonderful little characters) or rummynose tetras or harlequin rasboras (best schooling fish) would look very nice. I'd get about 8.
If you keep up with maintainance, you could also get a bristlenose pleco. These small plecs grow to a good size and look very unusual with bristles on their noses
They will also eat any algae.
The fish I suggested are quite easy to keep in a well-maintained tank but they are all quite small, active species. They do have interesting behaviours though and would be a nice tank to start off with.
Alternatively, you could try cichlids - a trio of yellow labs, some upside down catfish and a bristlenose pleco could make a nice set-up for your tank.
Something else to consider would be different species of rainbowfish. They come in so many different colors and each species is unique. You'll need to google those you are interested in - www.google.com
Obviously research the fish you are looking into and make sure you have correctly cycled your tank first - a 'cycle' may not be quite what your LFS said it was. Read the link in my signature to make sure you get off to a good start
Good luck and keep us updated - if you have any other fish in mind or don't like those I suggested, go ahead and tell us and I'm sure someone can suggest alternatives.
