You arent clear if you intend a 61.75 based on your existing foreign licence or a new issue of a private certificate by checkride. If you intend a 61.75 you dont need any tsa approval for float training, and you can add a float rating at the private level to a 61.75 but getting a 61.75 is in itself a fair amount of paperwork. I had a 61.75 and ended up doing the private checkride to get rid of it. I wish I had done that in the first place. There is a lot of bull spouted on here about visas and tsa. If you have any further ambition in faa licences my advice would be to apply for a b1/b2 visa, do the tsa and private checkride (on floats if you like) and be properly in the system. Makes it easier later for IR etc.