However the missiles will have difficulty locking-on to a LO jet like the T-50.
Maybe, maybe not. The T-50, unlike the MiG1.44 is primarily stealthy in the forward hemisphere, much like a Super Hornet. Indeed, it even uses a Super Hornet-like engine intake duct radar blocker rather than a serpentine duct with RAM as is used in the F-22 and F-35. T-50 is advertized as having a .1 to 1 meter forward aspect radar cross section. That's not bad, but not all that stealthy either. The EASA radar in the F-22, F-35, F/A-18, Rafale, Typhoon, Gripen, Su-35 etc, can detect an object with that size RCS at considerable range. And the passive sensors on many aircraft would likely pick up the T-50 at considerably greater range. I'm pretty sure that an AMRAAM's terminal homing radar is capable of locking onto an object with that size RCS, and that assumes it's intercepting head-on. From above and/or the side the RCS would be greater.