It will fail the MOT emissions test without the cat fitted, so you will need to swap back come MOT time.

As for the sensors, you will probably need to use the ST170 ones as the positioning of them is different so the wires are a bit longer. Either that or you could extend the wiring on your original sensors, which are currently positioned either side of the cat in its current location.

some of the early cars don't have a post-cat sensor, if this is the case, then fit the sensor to plug the hole, and chop the wiring off / tie it securely out of the way.