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I doubt I would go with 5 plus the large one.  Problem is goldfish eat a ton thus they produce a ton of waste.  It's great that it's full of plants and that helps a lot however the water can still become fouled very quick in such a small system.  You could probably get away with it if you were religious about doing water changes probably every week and by having a good filter.  Also do you have some form of fountain or stream into this pong to help it maintain it's oxygen levels?  This would be an absolute must for this many fish in a 25 gallon system.  You also have to remember because of plants and rocks a 25 gallon system isn't 25 gallons.  I honestly would feel safer to say 2 small goldfish plus the large one.  Again, it could be done with more but you risk having complications.  I hope this helps.