Fantails can grow to sizes larger than 6". As a general rule, most varieties of goldfish get large, and produce a lot of waste when they hit full size. The minimum gallons per tank should be 20 per fish, 30 would be better.
Remember with any fish ammonia and nitries should always remain at 0. Nitrates should;d be kept under 20 ppm for optimal health. With large fantail goldfish this will probably require large weekly water changes, even in a large tank...say 90 gallons.
I'm not sure what you mean by an algae waffler, but Im guessing you mean some kind of common pleco which should probably be left out of the tank. its a misconception they eat algae, what they for sure do is poop alot, adding extra strain to the tank bioload capacity. Also these guys can grow to 18" ( albeit it takes along time to get there) None the less Id avoid these guys anyway in a cold water tank like goldfish