In the case of the MPS6 & Gen 1 MPS3, you can run single cat, or catless without issues, you just need to put a small elbow where the second o2 sensor sits in order to fool the ECU, the gen 2 MPS3, requires 2 cats, as the program in the factory ECU is a lot less tolerant to changes.
As far as the legalities, in Australia it is illegal to run a catless exhaust, it is technically illegal to reduce the number of cats and mufflers fitted to the car from factory. Not sure how things are in NZ though.
If you are still finding issues with a CEL, a dashhawk or scangauge can clear them, as can most workshops with OBD readers.
Nearly all men can stand adversity. But if you want to test a man's character - give him power.
Abraham Lincoln