obx header installed and check engine light on

[yoinkAzenki]

i searched but couldnt find any straight answers so sorry if this has been covered. i just installed an obx header on my obd1 95 s14 ka24de and after driving around my check engine light came on. the car runs fine but the problem is that i have another o2 sensor fitting on the bottom of the header which is bigger than the stock fitting so my second o2 sensor won't fit. i decided to use an old o2 sensor that fits on the top of the header to plug up the bottom fitting. then i just plugged my second o2 sensor on but just left it hanging. so im guessing this is what made the cel go on. i pulled 3 codes from the ecu: 25 - Idle Air Control Valve, 35 - Exhaust Gas (EGR) Temperature sensor, 77 - Rear O2 sensor. after cleaning up the iacv i decided to erase the codes and the car still runs fiine. i know the rear o2 sensor is for diagnostic purposes but i wanted to know if its ok to run the car without the rear o2 sensor not properly installed? is there a 3 wire o2 sensor that will fit the bottom fitting of the header? i know the sr uses a 3 wire o2 sensor that will fit but is it the same as the rear o2 sensor because they look different at the tips? isnt the rear o2 sensor also the exhaust temp sensor? sorry for all the questions but any input would help i just want to make sure my car is running right.