I got two fish yesterday and would like some advice?

Yesterday I was at the fair and my family and I managed to win eleven little goldfish with the ball toss thing. We opted to trade in ten of the little fish for one large fish. So now I have a large shubunkin and some sort of little redhead/redtailed and white goldfish. The little fish "Nemo" has one fin smaller than the other (it probably was eaten) and the big fish is calico but doesnt appear sickly but whenever it swims the little fish does summersaults in its wake... Im very well aware of the quality and life expectancy of fair fish, however, i would like to try and create a happy environment while they last. I have them in a 5 or 8 gallon tank (its not full only 2 gallons because i ran out of spring water) I'm aware that one large goldfish needs at least ten gallons (am i right?) but until they reach the two week mark still alive I dont want to upgrade as I didnt even plan on having fish in the first place (my 5 year old cousin was supposed to take them home). I am ready though- as unplanned as this was- to take care of the fish for the next few years of their lives (if they make it) and would like to know what things to put in the tank to protect nemo from the big fish and if i can use a filter instead of live plants as i understand plants make it even messier as they will rot.