I agree that its a Corydoras paleatus (peppered cory, salt and pepper cory). I've seen some color variations in these. This is the darker type like yours, yes? http://www.tropicalfishandaquariums....pperedCory.JPG
On this page you can see that there is some variance in color intensity. The last pic looks most like yours, I think. http://http://www.planetcatfish.com/catelog/_species.php?species_id=271
Of course, a lot of the Corydoras can be difficult to distinguish from each other unless you're crazy experienced, which I'm not. But still, based on looks and availability, I'd bet its a peppered. (Who's a pepper? I'm a pepper!)
He would love (and needs) to have some more of his own kind. Cories are super social and need at least 5 of a kind if you've only one type, or at least 3 of each kind if you have multiple types of cories. Also, its ridiculously cute how excited they get to meet other peppered cories! I know you have a small space, so having any cories might not work for you.
You should read the Tropical Fish Profile for him (click on grey shaded name or use navigational bar at the top of the bag). You'll get info on his water parameters needs (which are very different than your livebearers), substrate needs, diet, etc. But, since your space is small, if you can't provide for him in either care, water parameters, or getting him peppered friends, I would suggest returning him. But, if you provide him with what he needs, then welcome to the world of corydoras! I think you'll love them!