They are funny birds indeed. Merchants will often say the same. Its especially odd from their perspective when they typically reorder and keep the same number per given sales tank - often with the same tankmates that also get reordered as needed with sales - and the success varies from week to week. Sometime they ignore each other, sometimes they try to kill each other :p Its not a size issue, they are slight indeed, but rather territorial by specimen at times.
I think the 30 gall is reasonable. I've had success spawning
magnifica and
decora but never saw a single fertile egg. I guess it wasn't rocket science to feed them like a holiday hog and then watch them spawn

I even had a pair of Helfrich's that were inseparable for years... but I never did see any indication of spawning or even being gravid.
Just focus now on maintain superb water quality and power feeding them.
IMO that means have adequate bolt holes but keep clutter otherwise to a minimum. Avoid the Chaeto for now too. A bit of a detritus trap when feeding fishes heavily... and also a small burden if you need to medicate, etc.
Bury some capped (one end)
PVC pipe lengths in the rockscape and lets start with that
