Yoour GH is fine, around 8 dGH (159 ppm which is the same as mg/l), so moderately soft/hard. The pH will likely remain roughly where it is.
The fish you have so far require cooler than "tropical" temperatures, and 74F is ideal for this, so any new fish need to have the same requirement. Temperature is very significant for fish because being ectothermic they rely on the temperature of the water to drive their metabolism and internal processes; even a degree or two can make a huge difference in their health.
The Giant Danio, Devario aequipinnatus [see below], is a shoaling fish that must be kept in a group due to its high level of interaction; 8 is considered the minimum number, so I would acquire another three. A four-foot tank would be better, but as you already have the five, increasing them for their benefit is better. Their activeness makes them unsuited to any slow or sedate fish, and they are best with medium/larger danio, peaceful barbs, and loaches; cories should be OK. Several sources mention their boisterous aggressiveness including fin nipping, and this will more likely be curtailed and confined to "play" if a group of 8 or more are kept. Given the tank space, this will fill the upper level.
Most of the more than 30 valid species in the Devario genus are unknown to aquarists. There are several species that attain the 4-6 inch size, and any of them may be offered under the common name of "Giant Danio." The species Devario aequipinnatus is the one most frequently encountered in the hobby. This species is similar to the more common Danio rerio [Zebra Danio], but larger and rather more boisterous. It has a lifespan of 5-7 years.
Like all danionins [a useful name for the species in Danio and Devario], it is a very active swimmer, preferring the middle and upper regions of the aquarium. These fish are jumpers, so a tight cover is a must.