Hi Amanda!
I agree that the heaters should be more horizontal. One reason being that the thermostat is located in the upper part of the heater body. As the heater heats the water, the warmer water rises. If the heaters are more vertical than horizontal, that warmer water will rise up and turn off the heater prematurely, or possibly cause it to turn on and off more than it should. The more the heater(s) turn on and off, the more wear it will have, leading to potential failures for being stuck on or off. Another reason, at least from what I believe, is that discus lay eggs on vertical surfaces. For juveniles this usually isn't an issue as they won't be mature enough to start spawning. However, as they get older and more mature, they are very likely to try and spawn on the heater itself. This will not only burn the eggs, but have a potential to burn and harm the fish as well.
With that said, I LOVE the current layout. I like the simplicity and the wood gives it a good dimension. If you're looking for some green color, I've seen a lot of folks be successful keeping Amazon Swords in plant pots with some root tab for nutrients. If you plan on doing something similar, I would wait until the QT period is over.
It looks like you've got a good feeding plan going on. My only suggestion, since you got 4" fish, would be to possibly increase the amount of water you're changing. For maximum growth, I would aim for 100%/day. This can be one large change, or split up into two separate 50% changes. Beef Heart mix is pretty messy, so I would say try and clean up after about 10-15 minutes of feeding it.
I love the color mixture you've got going! I'm looking forward to seeing how these guys progress.