not really sure, what you want me to do.

do you want to have number plates of 550i from Sixt?
example: M-GF 9443

btw, Sixt ONLY has 550i GT, NOT 550i sedane or estate.
and the chances that you are getting one of them is very low as is the chance to get a 535i
the 535i which Sixt has, are a bit longer already in the fleet and if there are not new ones coming, they will be out of the fleet by March already.

chances are much higher that you get an Audi A6 204PS or 245PS with Diesel engine or 525d or 530d

the assumption, you are making, that, because Sixt has 750i (yes, they do) Sixt also has 550i is a bit strange to me. until a few weeks ago, Sixt did not have the 550i GT and the 535i was the biggest available.