Put your credit card against your leg, is it touching your leg, has it cut your leg.
Do you use M8 bolts to go onto a M6 nut, of course not, you use the correct size.
Ive connected up my shift light recently in 20 seconds, that included wrapping the connection in insulation tape, and, there is no drop in volts from the ECU to the light.
Ive used these for 20 odd years and if you do the job correctly you wont have any problems.