Welcome, LT
Thanks for providing us with lots of helpful detail to try to get to the bottom of the problem. The fish certainly are suited to your type of water, you seem to have been doing everything right since introducing them, the water quality seems good and dechlorinated and tested with good products, etc.
Just to help us a bit more, as this seems a complicated situation to "diagnose":
* are there any other tank inhabitants, and, if so, what, and what size of tank are the fish in?
* what temperature is the tank at?
* how was the tank cycled - fishlessly with household ammonia and/or were there previously fish in the tank and/or was the filter media mature and from another tank? [Just wondering in the case of the latter two is there any possibility that the tank / filter media has been in contact with fish which haven't been well.]
* is the tank next to a heating source (eg radiator) or a TV/music speaker?
A few interim suggestions:
* try feeding them with standard flake food/pellets - they may feed better on something they recognise as food / are familiar with / had in the LFS before purchase, while brine shrimp and spirulina flakes may be unfamiliar and so may be better further down the line;
* add some more decor to the tank, so that they feel safer and more "at home" - they'll inevitably feel stressed in an unfamiliar environment, and it may just take a bit more time before they settle;
* keep a close eye on the fish and have a couple of broad-spectrum treatments (eg Aquarium Munster Medimor, eSHa 2000, Waterlife Myxazin or Protozin) on standby, in the event of more symptoms, so that you're ready to treat if need be;
* keep a close eye on water quality as you're doing, doing additional water changes if water quality deteriorates.
I have no experience with livebearer fish although I know they can already be pregnant on purchase, so am not familiar with whether they may display slightly odd behaviour in such situations. Hopefully someone else may be able to advise on this.