Since all you specified originally were festae and breidhori, and wanted spawning pairs, here's my take.
I have kept both, and many others.
I believe the festae at spawning maturity might be a bit crowded in that size tank, unless you had a perfectly copacetic pair.
My breidhori spawned in a similar size tank, at only about 5", and did fairly well until they grew too large, but in the mean time spawned another 2 or 3 times in that tank, raising lots of fry.
I almost always try to start with at least half dozen youngsters, and grow them up together, allowing them to choose there own partners. That doesn't assure you of a copacetic pair, and a few young hierarchal (less expensive) deaths should be expected, but I believe it ups the odds a bit. It does take more time and patience, but to me its better than buying expensive adults, and watching them kill each other.
Where I lived, I could usually pick up youngsters for $3-$5 each of both those species, and maybe cheaper at my local aquarium club auctions, where it was not unheard of to pick up a bag of 10 juvies of everything from some Vieja to dovii, for a buck or $2.
Boooooooring
