chickadee said:No I am afraid not. You see goldfish and Koi are coldwater fish and the others are all Tropical Fish and need heated and filtered water and would not do well with the coldwater fish. Plus goldfish take a lot of room. The general rule is 20 gallons for the first fish and 10 gallons for each additional fish. They are also quite dirty fish and produce a lot of waste and the Tropical fish would not do well in water that contains the level of waste products they put out. You need two tanks at the minimum one for the coldwater fish (goldfish and koi) and the other for the Tropical fish (silver dollar and pleco). These will both have to be quite large tanks as the goldfish and koi grow to be quite big and so does the pleco. The betta will need a tank of his own as he would not go well with any of the fish listed. They generally need a quiet tank with a few small peaceful fish like cories or otos to keep them company and do not do well in a community tank. The other fish either become aggressive towards them or they become the aggressor.
I hope this has been helpful.
well thanks for the help, my goldfish never grew bigger than what they are currently are so like you said i'll just keep an eye on them and as for the betta i'll go ahead and do thatpoefox said:Just keep an eye on size and stress; some goldfish don't grow that big. I'm sure you're doing a fine job looking after your fish. The betta probably would be happier (if you do get one) in say a 5-10 gallon tank with 3-4 little scavengers like otos or corydoras with him or her.