Rams can be tricky to keep alive. By rams, I mean blue, gold etc, but not Bolivian rams, which are a different species.
Rams have been bred in captivity for years. Individuals which have not had a wild caught fish in their ancestry for generations are less tricky. But breeders often introduce wild caught fish to improve the strain, and if any particular fish has a wild ancestor within a generation or two, they are more delicate. The problem with shop bought fish is that you don't know their ancestry, and neither will the shop.
Wild caught, or captive bred with recent wild ancestors, need very soft, acid water at 28 to 30oC. These fish cannot cope with med-hard water or a pH over 7. Temps below 28C slows their metabolism and their immune systems. They often suddenly decline and die after a month or two.
Fish that have not had a wild ancestor for many generations can cope with slightly harder, more alkaline water but it should not be very hard or over pH7. They can also cope with temps down to 26C.
Rams are also on the list of nitrate-intolerant fish - with your water parameters, at least your nitrate is OK for them.
Since you won't know if there are any recent wild caught ancestors in the rams in the shop tank, it is safer to assume there are and treat them accordingly.
Of the fish you have, cardinals can cope with the temps needed by rams. Neons, platies and rummy noses all prefer it cooler.
Rams don't have to be in a male/female pair but two males could fight over territory. Multiple females are a safer bet. And buying just a male and female doesn't mean they will automatically get on. These fish mate for life and are very picky when it comes to chosing mates.
Gold and 'ordinary' rams are the same species so there is no problem mixing the colours, subject to what I've just said about the sexes.
The alternative would be Bolivian rams. They can live quite happily at higher pH and hardness and prefer the same temps as your other fish. They don't look much in the shop tank, but they do colour up when they have settled in your tank. I have had some.
One thing I will warn you - the first part of the Latin name for both species is Mikrogeophagus, which translates as small earth eater. They eat by taking a mouthful of substrate and filtering it for food before spitting the substrate back out. With sand, this is not a problem (it even comes out through their gills). But gravel that is small enough to go into the mouth can be a problem, I learned that the hard way. My female Bolivian ram got a piece of gravel stuck in her throat. She got it out several days later but was never the same again. If you have large gravel that is too big to fit, that's fine. But if you have small gravel, be very careful. This is the reason I now have sand in all my tanks.