I will be installing engineered hardwood in my 2nd floor bedrooms. The hallway will remain carpet. This will be a floating floor install. I will be using baby threshold as transition piece between floor and carpet. How should threshold be installed. Should it be glued to subfloor, naild to subfloor, or possibly nailed to flooring even though it is a float install?

Any input appreciated

You never want to nail any mouldings to a floating floor. This will result in locking in the floor ,allowing no expansion space.

With baby thresholds I will glue it down with PL cartridge adhesive, then I will toenail through the carpet side of the threshold, ( carpet is hiding the nails ). If you like , you can just glue it ( if you think it will be secure enough). You can also top nail it if you like, making sure you dont nail into the hardwood. I try to avoid top nailing, for cosmetics.