I don't have the TRex 550, but I have a Blade 550 and I use a Castle Creations Edge 120HV on it. Bit of overkill but works well and it turns out this is now the ESC they ship with the combo kits. The stock ESC before this was an eflight 100A model. So a CC Edge 100 would probably work well too.
It looks like the 600MX and the motor I have on the 550X are similar specs. 75A continuous, 125A burst. If you're doing or plan to be doing hard collective or 3D with it, I'd go for the Edge 120HV of Edge 130 (non-HV). If you're going to be sport flying with moderate and only bursty power hits, then I'd go with an Edge 100.
I'm partial to CC. YEPs of similar rating would be fine too, though the built in governors probably won't be as good.