you could keep ONE colomesus asellus in a 15 g.
but i wouldn't keep it with anything you wouldn't mind it eating. i've only got confidence in mine because it completely
ignores the guppies i have spawning for it.
the thing about oddballs is that mostly they're considered odd because of their size, difficulty in keeping, day-night cycle, or feeding habits. probably the size issue will have the biggest effect on your choices.
many of the oddballs that will be suited for a 15g will be either sedentary, nocturnal, or rather plain-looking. these aren't all necessarily bad things, per se, but don't make for a great "show piece".
Some things you could try (and their disadvantages):
--Bristlenose Plecos (pretty much nocturnal, may turn territorial)
--Bumblebee Gobies (difficult to feed those tiny mouths, territorial)
--Cockatoo Chiclid (could only keep one, condition sensitive)
--German Ram Chiclid (could only keep one, condition sensitive)
--Peacock Spiney Eel (escape prone, if mis-labled it could quickly outgrow tank)
--S.A. Puffer (may or may not slaughter your fish, WILL NEED teeth-trimming)
--Dwarf Puffer (WILL FIN-NIP, may slaughter)
--Killiefish (i think i heard they have VERY short lifespans)
--Rainbow Fish (schoolers, can get quite large)
--Hatchet Fish (about as sturdy as neons, schoolers, prone to jump)
--Betta (a common "show piece", may or may not attack other fish, prone to jump)
--Dwarf Gourami (a bit prosaic, can only keep one really)
--African Butterfly (sedentary, prone to jump, HUGE mouth)
I reccommend that what you do is swing by the shops and see what you like. Ask the LFS:
--What do you feed them and do they eat it? (Can I see?)
--When did you get these?
--Do you get them in often?
--Do you know if they're wild caught?
--Would they be compatible with X, Y, or Z?
--How many should I get if I have a 15 gallon tank?
Take the answers and tell us what they told you.
You should probably just get the compatible fish you like best from the shop that told you the least amount of bull. Oh, and they get BONUS points for honestly saying they don't know and not making crap up. EXTRA BONUS points if they offer to look up the info for you.
if you just can't wait for feedback, a bristlenose pleco or one german ram or a few hatchet fish should be perfectly alright.
if you fall in love with a S. American Puffer (Colomesus Assellus), check how it's tankmates look and what they are. because that particular puffer has such quickly growing teeth, sometimes they just can't get a good bite in and that will spare your fish. but you will have to trim them eventually and then watch out!
i'd really like to hear how this works out for you.