Hi, 1. I have both 3G SIMS, fits nicely, needed some work to trim the edges for a perfect fit. 2. Needed an old phone (Nokia in this case) to access the SIM toolkit menu to configure the 007 mode so that you can switch SIMs easily via the N900's phone dialpad (001 dial+hangup for SIM #1, 002 dial+hangup for SIM #2) 3. I have no experience with T-Mobile SIMs, but technically it should work. No harm in trying since I believe this unit simply control the signal lines of the SIM. You should try it out and post your results. 4. As for the different sizes, mine was different size too. But the cut template was large enough to give good access to all the pin contacts to the minute microchip in the center of the SIM chip, shouldn't be a problem.