Yes a 15 gallon will be fine for a while if you keep a pair on their own, and they should breed in there. Longer term (as they get closer to full size) you might want something a little bigger but see how they get along. I honestly think the water volume/floor size would be big enough for their likely maximum sizes, but if they get touchy with each other more space would definately be recommended, with lots of shelter. And of course, if you successfully breed the fry to half an inch or so, you'll need to move them all to something twice the size, and start moving the fry out of that once they're over an inch if there is a lot of them.
So basically the answer is yes but be prepared for lots of moving about later.
An extra point is that they could be a bit shy on their own in that size tank without dithers. Get lots of hiding places, a good lid (cover) and subdued lighting to chill them out.
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