If you want to keep Glass as a Sky TV device and add a Stream puck, yes, you'll need Whole Home to run both. You can add both Whole Home and a Stream puck via the MySky app.
If you want to remove Glass TV from your account and use a Stream puck as the sole device for watching Sky TV subscription content on, you'd need to ask Sky to remove the Glass TV from your account as the primary device and to supply a puck as your primary device.. It will involve a phone call to Sky. Not sure what the timescales might be or the process for removing Glass & replacing with a puck as your primary device.
If you only want Sky Sports and don't need anything from the mandatory Sky Entertainment and Netflix tier, you'd be better off with NowTV; it is a cheaper means of getting Sports and you can use it on more than one device at a time if you have the appropriate Boost add-on.
Whole Home is a 31 day rolling contract, so your current subscription cost shouldn't change as a result of adding it on. If you go through the process of buying it and adding a puck, you'd should see a summary of your updated monthly costs before committing.
Sky Glass & Stream Puck customer w/Sky Entertainments & Netflix, Sky Cinema, Whole Home & UHD/Dolby Atmos add-on.
