Well I did 4 rolls in the shower, steamed it up good, closed the door and let it dry for 2 hours. When I first opened the door, there was slight cupping, but after about 15 minutes, it was business as usual. So I did a test, I souped 4 different kinds of film in the same mix of R09 1+50, HP5, Neopan SS, TMX and Tri-X all in 35mm. These were older rolls, so dev time did not matter, only being in the same soup did.
They ALL did the same damn thing! Tri-X was the worst, Neopan the flattest, but the differences were minor. Humidity was around 25%. Needless to say, I was ticked, ready to give up on 35mm. But then I followed some advice from earlier, before cutting the film, spooled it inside out and secured it for a day, it worked! All the negs are flat, good to go.
So this is the way I will do it from now on. As for the other 25 rolls that cupped, they live under heavy books until printing...