Wow, that's awesome! I love the look you've got going there. I'd be tempted to put the wood in the middle and have the cabomba on both sides. Unfortunately goldies are little lawnmowers...fortunately, cabomba is one of the best as far as fake plants go, a quality imitation (once it has some algae growing on it) is almost indistinguishable from the real thing
Those are small goldfish and they will be fine for a good while. I'd definitely recommend a 75 gallon or larger as their ultimate home, those are a common size and can be gotten quite cheaply. In the meantime, don't feed them too much, it will keep them from growing too fast
: Veggies, like zucchini and cucumber, and roasted seaweed (nori) are great low protein foods for goldies...and it's large quantities of protein that usually drive fast growth.