Tubifex is less of a food and more of a treat. Should be fed sparingly (I'm told it can be messy to feed, but I haven't experienced that in my tank), like bloodworms (can also get these from Hikari at PetCos & PetSmarts, similar container, red top).
You can keep the Koi if you want, but it will get to be about three feet. He's bigger than the other fish because he's well.. a Koi! A 30 gallon tank will just not do it for him. He will need a pond.
30 gallon - 55 gallon tanks are very cheap when gotten off of eBay, used. Frequently they also come with stands and filters and heaters (obviously not to be used with your Goldfish - they like cold water) hoods, lights, etc. My 38 gallon cost fifty bucks and came with gravel, a two tank stand, upgraded lighting, hood, filter, powerheads, and she drove it over for free.
It can't hurt to ask the seller if they'll include established gravel from the tank, without letting it dry out - this will cycle your tank for you very quickly, and you don't have to wait the 4-6 weeks cycling time before adding your fish to their new home.
30+ tank retail can cost a bit. The Tubifex cost me somewhere around five dollars. This is not needed, but can be a treat (should not be the main diet), and isn't very expensive.
They can stay in the tank for now, but they shouldn't. It's very unhealthy for them to stay there. Do you have a test kit? I'd be surprised if your Ammonia reading was zero. To help with this, up the water changes, and make sure to do your gravel vacs.
Happy Goldfish swim around the tank, and it doesn't sound like a five gallon is giving them much room to do that.