1) as long as it fulfills the requirements regarding ground clearance (>100mm), front and back arrival/departure angles, you do not decrease the damper stroke length by more then 1/3 travel and its not riding on the bump stops then you will be fine - to be safe, raise it to an amount greater then 100mm ground clearance - say 120mm for safety sake
2) no you do NOT need a permit for coilover suspension modifications AS LONG AS the suspension replacement parts were designed FOR THE CAR YOU ARE PUTTING THEM ON (ie retro fitting honda parts to put on your nissan is a no-no, technically so is putting silvia parts on your skyline or vice-versa, but putting coilovers designed FOR your model skyline on your skyline is fine)
if you want to get more info then read the National Code of Practice documents, VSB 14: NCOP11_Section_LS_Tyres_Suspension_Steering