I've been using a 6603 ROM almost the entire life of the phone, since T-Mobile USA doesn't provide updates (strange that T-Mobile in other countries are up to date...). No problems. The only thing that doesn't work is WiFi calling, which probably won't be an issue if the Z is your source of internet.
Mobile Hotspot works fine. It is a built in setting on the Z, no need for the T-Mobile app. T-Mobile's network does seem to know you are using it though, and will cut you off at your tethering limit (if you have one). Mine cuts off at 2.5 GB, which is all my plan includes for tethering. My old plan didn't include tethering, and it didn't work at all, even rooted and with various free tethering apps.
Just follow the instructions on this forum to make sure you retain the T-Mobile baseband when you flash the 6603 ROM.