I am looking to upgrade the lens on my canon 550d. An EF-S 18-135mm f/3.5-5.6 seems to be an all rounder for my needs. But i am thinking if there is any compatibility issue or such with a USM compared to an STM lens with the specific camera model.
Any Canon EF or EF-S lens made since the EOS system was introduced in 1987 is fully compatible with your EOS Rebel T2i/550D. This is true regardless of whether the lens in question uses a "micro-motor", USM, or STM to move the lens' focussing elements.
For the differences and advantages/disadvantages of using each type of lens, please see: