The help http://www.mkgmap.org.uk/doc/options says

--family-id=integer
    This is an integer that identifies a family of products. Range: [1..9999]
    Default: 6324 

I found no code that actually checks this limit.
Reg. your splitter options: It is probably best to use the corresponding value for splitter, so that tiles for the map with id 1032 start with 10320001 :–mapid=10320001