Hello Ghostdancing
Don't get bogged down by the 7% or 4% or the 85% that are all misquoted as "rules" but they are only guidelines. Whilst I knock the actual figures, I do agree with the principal of careful loading, which is factor to aid safer towing.
The law requires a nose weight to be a minimum of 4% of caravan mass or 25Kg which ever is greater and not exceeding the lower of the two limit figures quoted by the car and the caravan manufacturer.
In your case the upper limit is 50Kg (based on the figures you quote) The actual nose load is nothing to do with the weight of the trailer, but how the trailer is loaded. It is archived by positioning loads either in front or behind the trailers main axle until the desired load is created - see saw fashion.
50Kg nose weight is quite low, but that is the figure you must work with. It is generally accepted the best towing configuration is with the highest nose load you can obtain (within the limits) so you should aim for the 50Kg if possible.
The data abase I use does not cover the Roomster, so I cannot of any figures, but you should be able to check the allowed max trailer mass either from your handbook, or the dealer/manufacturer - Do not rely on third party sites, some do contain errors.
Many modern cars have a mass plate which carries four figures, they are all maximums, and relate to front axle rear axle, max mas of the car, and the max combined mass of car and trailer. In essence the difference between the max mass of the car and the combined mass, is the available mass for the caravan.
It is not immediately obvious why, but the trailers real nose load is not part of the trailers MTPLM, but is part of the cars load.
As for what you can put in the car, you will have to check the cars specification, but the ultimate limit will the axle loadings.
I hope this helps