My own tuya/smart_life account username is my phone number that I registered with. I know the tuya app said I could use my email address but only my phone number worked. (no country code, just the 10 digit phone number, no dashes dots or other stuff either)
I only have the username (my email) and password, I don’t have the platform data inputted (not sure if this changed since first implementation) and it works well
It might be defaulted to one of the platforms but when I first added my user info I got an error in the log when trying to access the wrong platform with the wrong credentials. Had to re-registered my account to the official Tuya one and adding Tuya as platform, then no problem what so ever.