6. I've read that people don't suggest  gravel/rocks at the bottom of the tank beacause they mess up the filtration and create problems for the fish. True or false?

1. Yes, they're fine together You could also add: Fantails, Pearl scales, Pom poms...
2. They're hardy and strong fish, but you need to be more careful to feed them a varied diet than the more "simple" varieties of fancy goldfish. You'll need high quality staple pellets, 2-3 different kinds. Aside from this, you should also feed them bloodworms, tubifex, krill, brine shrimp or gammarus a few times a week + vegetables every now and then You can buy dried seaweed for them, or you can give them shelled peas, blanched zucchini, lettuce, spinach, cucumber... (For more info, you could check out the "How to feed Veggies" thread)
3. During the cycling process, ammonia will spike during the first few days/weeks, this is highly toxic to fish, so it would be safer if you'd cycle the tank before adding the fish. As the beneficial bacteria will multiply and settle in the tank, the ammonia will turn into nitrite, which is also dangerous, but not at all as toxic. Finally, the nitrite will be exchanged to nitrates, and from now of you'll have to change the water every week to keep the nitrate levels healthy and safe for the fish
During cycling it's a good idea to add beneficial bacteria from a bottle every week, and add fish carefully, start with one fish and monitor the water quality closely in case a mini cycle would start. The biological system isn't used to having a constant source of ammonia (the source being the goldfish) so you'll need to perform small water changes regularly to keep the levels safe. Buy a test kit for pH, ammonia, nitrite and nitrate and test every day for the first few days. Later it'll be fine just changing half of the water weekly and testing only when you notice that the fish are acting strange.
4. I feed my goldfish with brands like Hikari, Tetra, JBL and Sera, all really good foods! There are special foods developed for ranchus and orandas if you're interested. I feed my fish three times a day in small portions (goldfish don't have stomachs so they can't store any nutrition, this is why they need many small meals during the day). In mornings and evenings I give them pellets, and for lunch they get live foods or vegetables
5. LIVE! Plastic plants will give a pretty unnatural and fake impression, and they could even have sharp edges that could cut your fishes' tails. If you'll get for example Java Ferns and different types of Anubias you can make a beautiful aquascape, and the goldfish love nibbling on the leaves and looking for food in the plants
! You can also try growing these plants onto pieces of driftwood, that looks really pretty! And they've got incredibly strong roots, so the fish wouldn't dig them loose.
6. The gravel needs to be a suitable size, pretty small pieces with even shapes are usually safest, so try finding some smooth, relatively round gravel in about pea size, and add a 2 inch layer or so in the tank, that will make planting easier, and it will make your tank look more natural. It's pure bullsh*t that the gravel would be any danger to the fish or filter. On the contrary, a part of the goldfishes' natural behavior is digging in the gravel for food particles. Personally, I find it cruel to keep them from living a natural goldfish life :/

Java Fern, by far! This plant will usually live all on its own, multiply and grow without you having to provide any extra plant nutrition or anything, not to mention that it tastes really bitter for the goldfish so they won't eat it

I have a forest of these in the back of my tank, and a smaller one growing on some bog wood, it makes a very nice look even if they're all the same plants
If you want something small too, get a bush or two with Dwarf Anubias, these will grow like a little carpet, out towards the sides, so they make really nice foreground plants!