29-30 would be good for just two. If you want to be really generous, a 40 gallon breeder tank would be wonderful and give you the option of adding a third fish at some point. If you have a choice, horizontal is preferred over vertical because it gives them a lot more surface area.
Supposedly they eat plants, but if you feed them really well there shouldn't be a problem. Most people way underfeed their goldies, which is probably for the best considering most people really overcrowd their tanks and don't clean them often enough.
Mine have duckweed floating on the surface. I have to keep a separate bucket of it growing all the time because they will eat it down to nothing, but it's really good for them. Especially deep bodied fancies like fantail and ryukin because they tend to get constipated on a pellet or flake diet.
I had a big gorgeous aponogeton and a lotus that were growing really well, but accidentally killed them. T.T There were some ich cysts so I did a salt treatment. The plants were taken out for the treatment but I wanted to kill the ich on them too so I did a quick bleach dip and got distracted by kids and forgot to take them out of the dip. :cry: They did fine with the goldfish though.
My fish get fed 4-5 times a day with home made gel food and bits of cooked beef heart and tuna. They get a 50% water change 3x a week, partly because they're fed so heavily but also because I have them a bit crowded in their tank right now (the outside fish came in for the winter).
Sponge filters are magical and really affordable too. They go a long ways toward keeping a tank nice and healthy.