this is my breeding tank which i'm winding down now ive had it and the 10G predessor running for a few years now.
The best way to get them breeding ive found is simply provide suitable breeding spots. In my case i used a couple of small clay plant pots (which came from cactus) covered in some suitable aquarium safe pebbles.
When the female breeds (about one a month) she will go into hiding for a week or so, only coming out to get food. Then one day you see her out of her spot with 40 odd wrigglers, darting about and hiding at the slightest disturbance.
The more fish you have in the tank the more she will be *****y and chase them off. Fun to watch for a while but generally annoys the rest of your fish.
A few cory's should be ok but since they are ground fish there are less places for them to run so make sure your breeding spots are at opposite ends of the tank so they can goto the other.
I also found the breeding female was much happier with lots of plants, thus giving her a better sence of safety.
I've also read that these cichlids, like others, feel more secure with "dither" fish. Ie a small group of tetra's or something else small which reasure them there are no predators about by their calming presence.
Cac's only live a couple of years, one of the reasons they breed so fast
, so after my second brood I decided to keep a couple growing up to replace losses. I also bought in another one to make sure i wasnt inbreeding too much.
But this 2 female 1 male dynamic wasnt too great for my 20G as you often had both females breeding at the same time with some inter brood genocide going on.
Expect your broods to go from 40 odd fry to about 20 - 10 survivors. After a while the female will let them go/loose interest in them and you probably won't see the survivor's again for months unless you seperate them into a fry only tank to grow up.
I had mixed sucess with a fry only tank. It had no furnishing, a sponge filter and was alot of work to upkeep for limited visual fun. Also the fry in that tank developed alot of genetic mutations for were not sellable to the LFS
The fry i grew in the actual breeding tank were fine, so i think the mother taking care of them actually leads to better stock. Or at least she weeds out the bad ones.
Overall one pair in your 15G will be fine, maybe even 2 females and one male if you want a better view of the more agressive side of these fish. But for rearing fry stick to one pair.