In theory you could mix regular goldfish with Apteronotus albifrons. Regular goldfish are essentially just large (very large!) barbs in terms of requirements and temperament, and do well up to 24 degrees C. When it comes to water temperature, Apteronotus albifrons has just enough overlap that both species could be maintained at ~23-24 degrees C. Goldfish do of course prefer hard, basic water with lots of carbonate hardness, which isn't what Apteronotus albifrons wants; it wants soft, slightly acidic to neutral water. But provided the water wasn't too hard, the Apteronotus albifrons should acclimate fine. (Keeping goldfish in soft/acid water is a very bad idea, so not on the table for discussion.)
Whether you'd *want* to mix regular goldfish and Apteronotus albifrons is an entirely different question. Goldfish are notoriously messy, schooling fish that stress the filtration system on even the biggest aquaria. I'd personally consider them more pond fish than aquarium fish. In any case, you'd be hard pressed to keep keep the good water quality Apteronotus albifrons demands in a tank stocked with a school of goldfish much less than 450 litres/100 gallons and a filter system offering not less than six times the volume of the tank in turnover per hour. You have a 200 gallon system, which I suppose does make the mix a bit more reasonable, but I'd still be checking water quality regularly to make sure the Apteronotus albifrons remains healthy.
Fancy goldfish really shouldn't be mixed with anything, even regular goldfish. There are simply too many variables re: swim bladder position, split fins, arched spines, etc. that make them difficult to combine with tankmates. At best, they lose out at feeding time; at worst they get badgered or nipped. Your turtles for sure will start nipping at their fins. Yes, red-ear sliders are primarily herbivores and shouldn't be eating meaty foods or pellets more than once or twice a week, but that doesn't stop them being opportunistic. A slow-moving fancy goldfish is simply too much of a temptation.