Welcome to Tropical Fish Keeping. Nice to have you with us.
My post will include some of what's already been suggested and expand a bit on a few points. You mention that you do know the eventual size of the BGK so that is good. The minimum tank you will need for this fish is a 6 foot (length) by 2 foot (width). Nothing less, as this fish will attain 20 inches and is inflexible so it will not be able to turn around in anything less than 2 feet width.
Guppies might survive now, but as the BGK grows they will disappear. Knifefish are predatory, and small fish will likely be eaten. An excellent tankmate is the angelfish, in a group of 5-6, and obviously in a larger tank than what you presently have. I'm looking to the near future with the 6-foot tank.
Discus also work well. In fact, these three fish do occur together in parts of their habitat. But of prime importance is the light, or rather lack of light, that all three prefer and the BGK demands. This is a nocturnal fish, and in a tank with overhead lighting that is not minimal and then reduced further with floating plants, they will be nervous and stressed, and that means unhealthy. As I mentioned, angels and
Discus like the same type of environment.
So this brings us to plants. With so little light, submersed plants will struggle. Suitable species are
Anubias,
Java Fern and
Java Moss. While not strictly Amazonian in origin, they do make good lower plants as they tolerate shade. All three attach to rock or wood, and the latter--wood--is something you want lots of with the BGK. Chunks of bogwood (the Malaysian Driftwood often available in stores is ideal) with tunnels and crevices will further settle the knifefish.
Substrate is basically decorative without substrate-rooted plants to deal with, so fine gravel or sand in a fairly shallow layer is best. I would opt for sand myself, a dark colour, something as simple as playsand will work. A layer of dried leaves (oak, beech, Indian almond) would be authentic.
Floating plants can be thick, and these will improve water quality which is also essential and keep nitrates low, while releasing a lot of oxygen through their roots into the water.
Water Sprite is perfect for this. [BTW, this name has shaded, meaning it is in our profiles, so you can click the name to see that profile. Profiles of fish and plants are under the second tab from the left in the blue bar across the top of the page.]
Light is critical for plants, but as we have mainly floating we can do with less. I don't know what fixture came with your present 60g, but a single T8 tube if it is fluorescent or two 10w or 13w Compact Fluorescent bulbs if incandescant (screw-in) will be fine. Tube/bulbs with a kelvin around 6500K is best for lant growth and true colour rendition.
Filter, I would go with a canister on tanks over 50g, one rated to the tank size is fine. The filtration will be adequate, and the flow will be variable. These fish do not like excessive water movement. For heaters, buy the best; no piece of equipment is as important as the heater. One that fails and either overheats or fails to heat the tank overnight can easily mean the loss of the fish. And for larger tanks like the 60g I suggest two heaters, one at either end. The filter should be setup with the intake at one end and the outflow at the other (lengthwise) to allow for a thorough movement throughout the tank, and a heater next to each works well. For a 60g I would suggest two 150w or 200w heaters.
I second the API liquid test kits. As this is a new setup, I would recommend the Master Combo, it has pH, ammonia, nitrite and nitrate; you will want these for the first several months at least, and pH and nitrates is useful after that. Water quality is critical for knifefish.
Hope this is of some help.
Byron.
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