A 15 gal, overfiltered, may work for a breeding pair of cons. But they won't have much room to get away from each other if things get ugly, so provide some cover (caves, fakes plants, etc..) and keep an eye on them.
My mixed CA cichlid tank, which includes two cons, will kill guppies, rainbow fish, and swordtails. So I would say no to the danios. A con, properly cared for and kept in good water, will not stay 'younger' for long, they grow big very quickly. You may try a female in a community tank, they don't get as big. But they are still very fast.