67 is a little cool, low 70's is better. It isn't real critical, you can wait a few days to add a couple more. One of the problems with a traditional cycle with fish is that you have to add them slowly, or your ammonia & nitrite levels will spike.
Hex tanks are taller, with less surface area per gallon. In a traditional rectangle 50 gallon you would have more surface area, which gives you more oxygen transfer to the water. With a rectangle 50 I would start with 5 danios, but since yours is a hex with less surface area 3 is better.
As far as chemicals go, the most involved thing you should be doing with chemicals is water tests, all you need to add to the tank water is a good dechlorinator. I use Prime, a Seachem product, as a water conditioner. That is all that gets added to my tank water, no pH adjusters, nothing else. It is all you need to add unless you get into breeding specific species, most any fish can be kept in tap water once properly acclimated. Don't get fooled by any lfs that tells you that you need to adjust your pH, it's nothing that needs to be fooled with, and will cause more harm than good.