Is adding aquarium salt okay for these fish?

No. Neons in particular are salt intolerant. The fish you have are true freshwater fish and do not need salt. Salt is used as an excuse for poor water quality. Keep water quality healthy and there is no need for it. And being alive doesn't mean they're happy, you may not care about the shoaling thing, but the fish do. When those Balas are big they may well eat the Neons too.