C41 fixers should have a controlled pH and should be very strong fixers. If you change from Ammonium Thiosulfate to Sodium Sulfate, you likely change pH and will have to readjust. And Sodium Thiosulfate is weaker than Ammonium Thiosulfate, so you might end up with retained silver.
If you insist on using Sodium Thiosulfate, start with Agfa 304 or Ole's OF-1 recipe and adjust pH to 6.5 with Sodium Carbonate and Sodium Bisulfite. Consider two bath fixing if you detect retained silver.