Not sure about the bike rack number plates, prob a bit of a flaw in the system there, but i'm sure they could get you on it somehow, best to just get the slim plates and not worry about it.
I couldn't be bothered paying $170 or what ever it is a year for custom plates so i ended up getting R and S series plates, think it was $70 for ever, and i can swap to what ever car i want. Works out well with the RB and SR.