Ok, good question. Go by the directions in the initial post. They are worded better than I can paraphrase.
It says: "b) Install your current OS on your BlackBerry device to your PC.
If you are using a beta OS, it must be installed on the PC. Find the most
current OS release for your model in this same 9500/9530 model section, in the Sticky Notes at the top."
So you're right, you don't download that Tmobile... download it from the link in that line of the instructions.
You have 5.0.0.1015 installed on your 9550 now. Click on the latest OS 9550 thread at the top of the page, that will be OS .1082, scroll to the bottom of that initial post to the Previous OS here line, and find .1015.
Download it, install and continue.