Difficult to sex without an experienced eye and not always that accurate.
Generally Cardinal fish are territorial with various degrees of aggression dependent upon species. Orange Stripes have moderate tolerability for their conspecifics. Orange Stripes really only shoal when they are small juveniles.
I would suggest that a 40g would be a minimum for two pairs.
A single or pair would be a nice fit for your tank. Remember they can get up to 2.5 / 3.0 inches in aquariums.
There are other Cardinal options out there, even though the Orange Stripes are a very nice fish.
You can keep other peaceful reef fish with the Cardinals in your tank. You could keep gobies, sleepers, blenies, flasher wrasse, dartfish, etc.