Here's a care sheet that's got some pretty good detail. Rock, wood, etc are all good decor options. They're very forgiving of water chemistry and will do alright in most conditions; but somewhere in the area of moderately hard and slightly alkaline (think low to mid 7's) is probably ideal.
I will warn you though that a 15 gallon is going to be too small pretty quickly. They're not super small fish (somewhere in the 3 to 6 inch range) and, being cichlids, are fairly aggressive (meaning you can have big issues if you try to crowd them).
1) Cichlids are often difficult to sex reliably. There are some general indicators (body shape, fin shape, minor colour differences); but it's not 100%
2) A male plus a female does not a pair make. There are no guarantees that they'll like each other.
3) The caresheet I linked to suggests something half again as long as a standard 15 gallon for a pair (although it does mention you could go a little bit smaller for a breeding pair). When you throw it into AqAdvisor it tosses back bunch of warnings. Both of these jibe pretty well with my gut feeling of this probably not being a good idea.
4) As a general rule of thumb, you *really* don't want to crowd most cichlid species (excluding East Africans). They're a bit fighty at the best of times.
All in all, lots of opportunities for Murphy's law here. If it were me I'd be a lot more comfortable with a 40 gallon breeder.